GIFT  OF    - 

;nter  of 

illiam  Stuart   Smith 


\±t*s~4* 


THE  INCANDESCENT  LAMP 


AND 


ITS  MANUFACTURE. 


BT 


GILBERT    S.    RAM,    A.I.E.E. 


NEW  YORK : 
THE    D.     VAN     NOSTRAND    COMPANY, 

23,  MURRAY  STREET,  AND  27.  WARREN  STREET. 

LONDON : 
THE    ELECTRICIAN"    PRINTING  AND   PUBLISHING   COMPANY, 

LIMITED, 
SALISBURY  COURT,  FLEET  STREET,  E.C. 

1894. 
[All  Rights  Reserved.] 


\ 


Printed  and  Published  by 

THR  RLRCTRIOIAN"  PRINTING  AND  PUBLISHING  CO.,  LIMITED, 

],  2,  and  3,  Salisbury  .Court.  Fleet  Street, 
London,  E.C. 


CONTENTS. 


PACK 

LIST  OF  ILLUSTRATIONS    vii. 

INTRODUCTORY  ix. 

CHAPTER   I. 

THE   FILAMENT 1 — 5 

Carbon  Filaments.  —  Filaments  of  other  Substances  than 
Carbon.  —  Volatilisation  of  Carbon.  —  Disintegration  of 
Carbon. — Emissivity  of  Carbon. — Methods  of  Preparing 
Carbon. 

CHAPTER  II. 

PREPARATION  OF  THE  FILAMENT 6 — :19 

Swan's  Process  of  Parchmentising  Cotton  Thread.— Drying 
Prepared  Thread.— Draw  Plates  for  Cutting  Thread. — 
Squirted  Filaments.  —  Wynne  and  Powell's  Process. — 
Weston's  Process.  —  Furfurol  Process.  —  Cuprammonia 
Process. — Other  Processes. 

CHAPTER  III. 
CARBONISATION  21 — 32 

Precautions  necessary  on  account  of  Shrinkage.— Filaments 
Carbonised  on  Blocks.— Double-loop  Filaments.— Method 
of  Packing  Filaments  in  Crucible.  —  Flat  Filaments. — 
Construction  of  Furnace.  —  Pyrometer.  —  Precautions 
during  Carbonisation. — Unpacking  the  Crucibles. 

CHAPTER  IV. 

MOUNTING 33 — 44 

Use  of  Platinum. — Various  Methods  of  joining  the  Filament 
to  the  Wires.  -Mechanical  Joints.— Deposited  Carbon 
Joints. — Socket  Joints.  — Butt  Joints. — Apparatus  and 
Details  for  making  Deposited  Joints. 

A 


870733 


IV.  CONTENTS. 

CHAPTER   V.  PAGE 

FLASHING 45—67 

Uses  of  Flashing.— Flashing  to  Resistance  (hot  or  cold). — 
Effects  of  Flashing. — Flashing  in  Gas  at  Atmospheric 
Pressure.  —  Vacuum  Flashing.  —  Gasoline  or  Pentane 
Flashing. — Quality  of  Deposited  Carbon.— E.M.F.  required 
for  Flashing. — Extent  of  Surface  with  Flashed  and  other 
Carbon. — Effect  of  Reduction  in  Resistance  due  to  Flashing 
on  the  Voltage,  &c. 

CHAPTER   VI. 

SIZES  OF  FILAMENTS  (UNFLASHED) 69 — 79 

Circular  Filaments. — Square  Filaments. — Flat  Filaments. — 
Hollow  Filaments. 

CHAPTER   VII. 

SIZES  OF  FILAMENTS  (FLASHED) 81 — 97 

Curves  showing  Sizes  Before  and  After  Flashing. — Filaments 
with  Resistance  Cold  Double  the  Hot  Resistance. 


CHAPTER   VIII. 

MEASURING  THE  FILAMENTS    99 — 102 

Screw  Micrometer  Gauge. — Trotter  Gauge. — V-slot  Gauge  — 
Optical  Method. 

CHAPTER  IX. 

GLASS  MAKING 103 — 108 

Glass  Furnace. — Crucibles. — Making  Lamp  Bulbs  and  Tubing. 

CHAPTER   X. 

GLASS  BLOWING   109 —  1 1 9 

Cannon  Blow-Pipe. — Large  and  Small  Flames. — Precautions  in 

Working  Lead  Glass.— Compound  Blow-Pipe  — Air  Supply 

for  Blow-Pipes. — Lamp  Bulbs  Blown  from  Tubing. 

CHAPTER   XI. 

SEALING-IN    121—129 

Description  of  Process. — Annealing. — Spotted  Bulbs. — Cutting 
Glass-Tubing. — Grinding  Stoppers,  &c. 


CONTENTS.  V. 

CHAPTER  XII.  PAGE 

EXHAUSTING 131 — 177 

Necessity  for  Exhausting. — Mercurial  Air-Pump. — Pumps  of 
Geissler,  Laue-Fox,  Toepler,  Swinburne,  Sprengel. — Pumps 
Arranged  for  Factory  Work. — Methods  of  Lifting  the 
Mercury. — Shortened  Pumps. — Weston's  Pump. — Sawyer- 
Mann  Pump  Room. — Kennedy's  Pump. — Application  of 
Current  to  Lamps  during  Exhaustion.— Heating  the 
Lamps. — Air  Film  in  Pumps. — Method  of  Constantly 
Maintaining  Vacuum  in  Pump. — Moisture  in  Lamps  and  in 
Pumps. — Method  of  Joining  the  Lamps  to  the  Pumps. — 
Pump-Room  Blow-Pipe. — Testing  the  Vacuum. — McLeod 
Vacuum  Guage. — Induction  Coil  Test. — Clearing  and 
Distilling  Mercury.  —  Mechanical  Pumps.  —  Giminghain's 
Pump. — Rotary  Pump. 

CHAPTER   XIII. 

TESTING 179 — 192 

Testing  by  Eye. — Photometer  Testing. — The   Photometer. — 

Wattmeter. — Meaning  of  Candle-power.—  Candle-power  of 

Flat  Filaments. 

CHAPTER   XIV. 

CAPPING     1 92 — 194 

Caps. — Methods  of  attaching  by  Plaster. 

CHAPTER   XV. 

EFFICIENCY  AND  DURATION  195 — 201 

Temperature  and  Efficiency. — Falling-off  in  Candle-power. — 
Effect  of  Variation  in  Pressure  on  the  Candle-power. — 
Causes  of  Falling-off  in  Candle-power. —Effect  of  Fall  in 
Resistance. 

CHAPTER   XVI. 
RELATION  BETWEEN  LIGHT  AND  POWER 203 210 

Tests  of  Four  Lamps. — Calculation  of  Candle  power  at  any 
Voltage. 

INDEX  TO  CONTENTS     213 


LIST  OF  ILLUSTRATIONS. 


FIG.  PAGE 

1.  Apparatus  for  Parchtneutising  Cotfcou  Thread  (Swan's  Process)  8 

2.  Glass  Guide  Rod  for  Thread         ...  10 

3.  Method  of  Drying  Parchmentised  Thread          ..           13 

4.  Circular  Jewelled    Draw-Plate    for    Cutting    Parchmentised 

Thread 15 

5.  Split  Jewelled  Draw-Plate  for  Cutting  Parchmeutised  Thread  15 

6.  Carbon  Frame  for  Carbonisation  of  Single-Looped  Filaments  ..  22 

7.  Carbon  Frame  for  Carbonisation  of  Double- Looped  Filaments  23 

8.  Carbonising  Furnace          .......         ...         .            ...          ..  26 

9.  Pyrometer  for  Indicating  Temperature  during  Early  Stages  of 

Carbonisation ..  27 

10.  Early  Swan  Lamp,  showing  Crayon-holder  Joint         ..           ...  35 

11.  Early  Lane- Fox  Lamp,  showing  Carbon  Tube  Joint 36 

12.  Early  Maxim- Weston  Lamp,  showing  Bolt  and  Nut  Joint  36 

13.  Wire-Twisting  Machine                             37 

14.  Cementing  Machine  for  Socket  Joints    ...          ...          ..  39 

15.  Cementing  Machine  for  Butt- Joints         .                       ..:'         ...  40 

16.  Apparatus  for  Coal-Gas  Flashing 52 

17.  Apparatus  for  Pentane  Flashing  ...                      ...         ...          ...  55 

18.  Apparatus  for  Pentane  Flashing  ..           55 

19.  Clip  for  Holding  Filament  during  Flashing       56 

20.  Vacuum  Connections  for  Pentane  Flashing       ...          ...         ...  57 

21.  Curves   showing   Effect   of   Flashing   on    the   Resistances    of 

Filaments  of  Various  Diameters       ...           65 

22.  Curves   showing  Effect  of   Reduction  in   Resistance,  due   to 

Flashing,  on  the  Voltage  of  Filaments  of  Various  Diameters  65 

23.  Curves    showing  Effect   of   Reduction  in  Resistance,  due  to 

Flashing,  on  the  Current  for  Filaments  of  Various  Diameters  66 

24.  Curves  showing  Relation  between  Current   and  Diameter  of 

Filaments  for  Different  Durations  of  the  Flashing  Process. 
The  Diameters  represent  the  Measurement  of  the  Fila- 
ments before  being  Flashed 82 

25.  Curves  showing  Relation  between  Current  and  Diameter  of 

Filaments  for  Different  Durations  of  the  Flashing  Process. 
The  Diameters  represent  the  Measurement  of  the  Fila- 
ments after  being  Flashed  ...  ...  ...  ...  ...  83 

26.  Micrometer  Gauge  for  Measuring  Diameter  of  Filaments        ...  99 

27.  Trotter's  Micrometer  Gauge         ..           ...         ...         ...         ...  100 

28.  V -slot  Micrometer  (Jauj^e..           101 

29.  Glass  Crucible  for '•  Cone  "  Furnace       104 


viii.  LIST    OF    ILLUSTRATIONS. 

FIO.  PAGE 

30.  Glass  Crucible  for  Small  Furnace            105 

31.  Bulb  Mould 107 

32.  Cannon  Blow-Pipe 110 

33.  Form  of  Large  Flame  of  Cannon  Blow-Pipe      Ill 

34.  Pointed  Blow-Pipe  Flame ...  Ill 

35.  Compound  Blow-Pipe        ...          ...         ...         ...         ...         ...  113 

36.  Form  of  Flame  of  Compound  Blow-Pipe             ..'         ...         ...  114 

37.  Different  Stages  in  Blowing  a  Bulb  from  Glass  Tube  ...           ..  117 

38.  Different  Stages  in  the  Process  of  "Sealing-in  "  the  Filaments  122 

39.  Lamp  with  Annealing  Cap         "...           ..         ...                      ..  124 

40.  Geissler  Pump         .  134 

41.  Lane-Fox  Pump     ...                     ...                      ...  137 

42.  Toepler  Pump         ...         138 

43.  Safety  Side  Tube  Arrangement  for  Use  with  Geissler  Class  of 

Pump .'.         .f.          ...  141 

44.  Swinburne  Pump    ...          ...          .            ...         ..           ...          ...  142 

45.  Sprengel  Pump.     Simple  Form    ...         ...         ..                       ...  144 

46.  Sprengel  Pump  Distributor  for  Six  Fall  Tubes              145 

47.  Sprengel  Pump  Collector  for  Six  Fall  Tubes      146 

48.  Sprengel  Pump  with  Air- Pressure  Lift •••  i49 

49.  Shortened  Form  of  Sprengel  Pump         ...  150 

50.  Weston  Pump         ..  153 

51.  Pump  Room  of  Sawyer-Mann  Lamp  Factory,  1889      ...          ...  154 

52.  Swinburne  Pump.     Factory  pattern       ..  155 

53.  View  of  a  Pump-Room  with  Swinburne  Style  of  Pump           ...  156 

54.  Details  of  Top  of  Mercury  Reservoir  of  Pumps  in  Fig.  53       ..  157 

55.  Kennedy's  Pump    ...         ...         ..           ...         ...   .      ...         ..  159 

56.  Swinburne's     Arrangement    for    Constantly     Maintaining    a 

Vacuum  in  a  Pump     ...          ...          ...           ..         ...          ...  163 

57.  Blow-Pipe  for  Pump  Room           ...         .,          ...  169 

58.  McLeod  Vacuum  Gauge    ...                     ...         ...          ..  .         .  170 

59.  Mercury  Still           ...  175 

60.  Electrical  Connections  for  Testing  Lamps  on  Photometer       ...  187 

61.  Evershed's  Wattmeter  for  Lamp  Testing.      Sectional  elevation  188 

62.  Evershed's  Wattmeter  for  Lamp  Testing.      Sectional  plan     ...  188 

63.  Connections  of  Evershed's  Wattmeter    ...           .                       ..  189 

64.  Curve  showing  Percentage  of  Normal  Candle-Power  at  any 

given  Percentage  of  the  Normal  Volts          198 

65.  Curves  showing  Relation  of  Candle-Power  and  Watts  for  Four 

Lamps,  A,  B,  C,  D       205 

66.  Curves  showing   Relation   of    Candle-Power   and    Watts    for 

Lamps  B  and  D,  up  to  their  Breaking  Points         ...           ..  205 

67.  Diagram  showing  Relation  of  the  Logarithms  of  the  Candle  - 

Powers  to  the  Logarithms  of  the  Volts  and  of  the  Watts 

respectively  of  Lamps  B  and  D         ...         ...         ...         ...  207 

68.  Lower  part  of  Curves  of  Candle-Power  and  Volts  for  Lamps 

BandD             208 

69.  Curves  of  Candle-Power  and  Volts  for  Lamps  B  and  D  up  to 

their  Breaking  Points ...  209 


INTRODUCTORY. 


WITH  the  expiration  of  Edison's  master-patent  for  the  carbon 
incandescent  electric  lamp,  the  attention  of  electric  light 
engineers,  as  well  as  of  all  those  who  use  the  light,  is  once 
more  directed  to  the  consideration  of  the  lamp  itself,  to  the 
possibility  of  obtaining  better  lamps,  and  to  the  probable 
reduction  in  price  which  will  naturally  follow.  Owing  to 
the  long  prevailing  monopoly  in  the  sale  and  manufacture, 
there  has  been  little  inducement  for  those  interested  to  ex- 
periment and  to  study  the  problems  connected  with  the  incan- 
descent lamp.  As  a  result  of  this,  the  literature  of  the  lamp  is 
very  scanty,  and  is  entirely  confined  to  the  pages  of  the  leading 
technical  journals.  While  dynamos,  alternators,  transformers, 
arc  lamps,  and  almost  every  piece  of  apparatus  connected 
with  electrical  engineering  and  lighting,  have  been  written  on 
at  length  and  discussed  at  meetings  of  scientific  societies,  the 
incandescent  electric  lamp,  which  has  been  the  chief  cause 
of  the  very  existence  of  these  machines  and  apparatus,  has 
been  comparatively  neglected.  With  the  exception  of  the 
valuable  series  of  articles  by  Mr.  Swinburne  which  appeared 
in  The  Electrician  six  years  ago,  no  comprehensive  or  detailed 
account  of  lamp  manufacture  has  appeared.  The  manufac 
ture  of  the  incandescent  lamp  and  the  principles  underlying 
it  are,  consequently,  but  little  known,  except  to  those  actually 
engaged  in  the  work. 


X.  INTRODUCTORY. 

As  a  thorough  understanding  of  the  lamp  and  the  possi- 
bilities of  its  improvement  can  only  be  obtained  by  consider- 
ing the  various  processes  of  its  manufacture,  it  is  probable 
that  a  work  on  the  subject  at  the  present  time  will  be 
welcomed  by  those  who  are  interested  therein  and  have  not 
had  the  opportunity  of  studying  it  for  themselves. 

As  writers  in  whose  hands  this  most  interesting  subject 
might  have  fared  better  have  not  essayed  to  undertake  the 
task,  the  Author  asks  the  indulgence  of  readers  for  the  many 
shortcomings  which  may  be  apparent  to  them.  This  indulgence 
will,  he  feels,  be  the  more  readily  extended  to  him  when  those 
interested  in  the  subject  understand  that  "  The  Incandescent 
Lamp  and  Its  Manufacture "  does  not  profess  to  at  all 
exhaust  the  subject,  or  to  describe  nearly  all  the  processes  of 
manufacture.  All  that  is  attempted  is  to  give  readers  such 
information  as  the  Author,  in  the  course  of  a  considerable 
experience  in  lamp-making,  has  acquired,  and  to  place  this 
information  before  them  with  as  little  mathematical  embellish- 
ment as,  under  the  circumstances,  is  possible. 


CHAPTER  I. 


THE     FILAMENT. 

THE  light-emitting  portion  or  burner — called  by  Edison 
the  filament — of  the  incandescent  lamp,  it  is  well  known,  is 
composed  of  carbon,  which  is  raised  to  a  state  of  incan- 
descence by  the  passage  through  it  of  a  current  of  electricity. 
Carbon  is  not  the  only  substance  which  can  be  used.  The 
number  of  possible  materials  is,  however,  very  small,  and  for 
two  reasons.  Firstly,  because  the  material  must  be  capable 
of  withstanding  a  very  high  temperature;  and  secondly, 
because  it  must  be  a  conductor  of  electricity.  These  two 
•conditions  exclude  nearly  everything  except  carbon  and  some 
of  the  metals.  Of  the  metals  which  are  sufficiently  plentiful, 
platinum  alone  possesses  the  desired  qualifications  to  a  degree 
which  renders  its  use  possible.  Platinum  has  a  very  high 
melting  point,  and  will  stand  a  very  high  temperature,  but 
the  temperature  necessary  for  an  economical  incandescent 
lamp  burner  is  far  above  that  even  of  the  melting  point  of 
that  metal. 

The  incandescent  electric  lamp,  though  a  great  advance 
upon  most  other  artificial  sources  of  illumination,  gives  us  at 
best  only  a  very  inefficient  method  of  obtaining  light.  That 
is  to  say,  the  proportion  of  the  luminous  radiation  to  the  total 
radiation  is  very  small.  When  the  temperature  of  a  filament 
is  raised  by  the  expenditure  in  it  of  more  and  more  energy 
it  becomes  increasingly  efficient.  The  efficiency  which  can  be 
attained  is,  however,  limited  by  the  temperature  which  the 
filament  will  stand.  Carbon  will  stand  a  temperature  higher 
than  the  melting  point  of  platinum,  and  it,  therefore,  con- 
stitutes a  more  efficient  burner. 


2  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

Certain  metallic  oxides  possess  in  a  remarkable  degree  the 
power  of  resisting  very  high  temperatures.  It  has,  in  conse- 
quence, been  proposed  to  utilise  these  in  some  way,  and  mention 
[<$  them  jiais^eeii  made  in  some  of  the  earliest  patents  for  elec- 
tric^ lanrjss.  Here,  .however,  the  difficulty  occurs  that  metallic 
•  oxides  arfc  daq^-js^ndjq^tors  of  electricity,  and,  consequently, 
cannot  be°  used  alone.  They  can  only  be  used  in  conjunction 
with  some  conductor — either  a  metal  or  carbon.  The  Author 
has  tried  coating  both  platinum  and  carbon  with  oxides  such 
as  alumina,  magnesia,  lime,  &c.,  but  has  always  met  with  the 
difficulty,  in  the  first  place,  that  the  coefficient  of  expansion 
of  the  oxide  is  different  from  that  of  the  carbon  or  metal,  and, 
consequently,  the  coating  cracks  and  falls  off. 

It  has  been  pointed  out  by  Prof.  S.  P.  Thompson  that  the 
temperature  of  the  crater  of  the  arc  in  an  arc  lamp  is  con- 
stant, and  is  the  temperature  of  the  volatilisation  of  carbon. 
Any  other  substance  contained  in  the  carbons  volatilises 
instantly  when  reaching  the  arc.  The  carbon  itself  volatilises 
at  that  temperature,  and  all  other  substances  appear  to  do 
the  same  either  at  that  temperature  or  below.  This  being 
the  case,  it  would  appear  futile  to  attempt  to  increase  the 
durability  of  arc  lamp  carbons  by  the  addition  of  any  foreign 
substance. 

It  is  possible,  however,  that  some  substance  other  than 
carbon  may  yet  be  used  successfully  as  the  light-emitter 
in  an  electric  lamp.  Even  if  all  metallic  oxides  are  dis- 
persed at  a  temperature  less  than  that  of  the  arc  when 
used  in  conjunction  with  carbon,  they  are  not  all  necessarily 
volatilised  at  that  temperature.  The  volatilisation  may  be 
simply  a  secondary  effect,  the  oxide  having  been  first  reduced 
by  the  carbon.  It  does  not  appear  to  be  impossible  that  some 
oxide  or  mineral  substance  may  be  found  to  be  stable  at  tem- 
peratures higher  even  than  that  of  the  arc  if  there  be  no 
carbon  present.  The  great  trouble  in  using  any  such  sub- 
stance in  the  same  manner  as  the  filament  of  an  ordinary 
incandescent  lamp  is  that  of  electrical  conductivity.  It  is 
possible  that  some  of  them  might  consent  to  conduct  suffi- 
ciently if  subjected  to  a  very  high  E.M.F.,  such  as  can  be 
produced  by  transformers.  We  have  seen  quite  recently 
slate  pencils  used  in  the  place  of  carbons  in  the  arc 


THE    FILAMENT.  3 

slate  being  considered  an  insulator  at  ordinary  electrical 
pressures.  An  incandescent  lamp,  however,  with  such  a 
filament,  if  ever  constructed,  would  hardly  be  suitable  for 
domestic  use.  Again,  although  mineral  substances  when  mixed 
with  carbon  are  all  reduced  or  volatilised  in  the  arc,  such 
mixtures  may,  nevertheless,  be  used  for  incandescent  lamp 
filaments  at  a  temperature  much  below  that  of  the  arc. 

Great  improvements  may  yet,  however,  be  looked  for  in 
the  carbon  itself.  It  is  well  known  that  carbon  prepared 
by  some  processes  will  last  much  longer  than  that  prepared 
by  others.  The  efficiency  of  a  carbon  depends  upon  the  tem- 
perature at  which  it  can  be  run.  The  higher  the  temperature 
the  greater  the  efficiency.  The  limit  to  the  temperature 
possible  is  that  of  the  volatilisation  of  carbon.  This  tempera- 
ture in  the  case  of  an  incandescent  lamp  is  probably  lower 
than  that  of  the  arc.  The  temperature  of  volatilisation  of 
carbon  in  vacuo  is  likely  to  be  much  less  than  at  atmospheric 
pressure.  We  can,  therefore,  never  hope  to  get  the  carbon  of 
an  incandescent  lamp  anything  like  as  bright  as  the  crater 
of  an  arc  lamp. 

Long  before  the  temperature  of  volatilisation  of  carbon  in 
vacuo  is  attained,  there  is  another  effect  to  be  considered  which 
puts  a  far  lower  limit  upon  the  temperature  which  can  be 
maintained  in  practice.  Apart  from  volatilisation  properly 
so  called,  there  is  an  action  going  on  by  which  particles  of 
carbon  are  dissociated  from  the  filament  and  thrown  off  upon 
the  glass  globe  (which  is  usually  called  the  bulb)  of  the  lamp. 
This  action,  unfortunately,  begins  at  a  comparatively  very  low 
temperature — a  little  above  red  heat — and  increases  very 
much  at  higher  temperatures,  so  that,  although  the  filament 
may  be  far  below  the  temperature  of  volatilisation,  yet  it  is 
fast  falling  to  pieces,  and  the  bulb  is  becoming  greatly 
obscured  by  the  black  deposit  of  these  particles  of  carbon.  It 
is,  then,  in  overcoming  this  lower  limit  to  the  temperature 
that  improvements  must  first  be  made.  Some  carbons  are 
much  better  than  others  in  this  respect.  The  best  carbon  is 
that  which  at  the  highest  possible  temperature  disintegrates 
at  the  slowest  rate. 

Much  has  been  said  and  written  about  the  so-called 
efficiency  of  surface,  or  emissivity,  of  different  varieties  of 

B  2 


4  THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

carbon.  Some  varieties  have  a  greater  emissivity  than  others 
in  the  proportion  of  about  3  to  2.  That  is  to  say,  when 
running  at  the  same  efficiency  (watts  per  candle),  two  carbons 
of  precisely  the  same  dimensions  may  be  giving  different 
amounts  of  light  by  as  much  as  the  above  ratio.  Or  two 
carbons,  one  having  only  two-thirds  of  the  surface  of  the 
other,  may,  when  at  the  same  efficiency,  be  giving  an  equal 
amount  of  light.  It  has  sometimes  been  supposed  that  the 
one  with  the  smaller  surface  is,  therefore,  better  than  the 
other.  The  reverse,  however,  is  usually  the  case.  The  fila- 
ment with  the  smallest  emissivity  is  generally  the  best.  A 
high  emissivity  usually  means  a  dead  black  surface,  a  low 
emissivity  a  polished  white  one.  The  dull  black  surface  is 
generally  a  softer  carbon,  and  one  which  disintegrates  more 
quickly  than  the  other.  The  only  advantage  in  the  high 
emissivity  is  that  the  filament  is,  therefore,  shorter  and  may 
be  accommodated  in  a  smaller  bulb.  The  size  of  the  bulb, 
however,  should  not  be  determined  by  the  length  of  the 
filament  so  much  as  by  the  candle-power.  A  large  candle- 
power  lamp  may  have  a  short  filament,  but  it  should  not  for 
that  reason  be  put  into  a  small  bulb.  These  matters  will  be 
further  dealt  with  in  a  subsequent  chapter. 

The  chief  aim  of  the  lamp-maker,  next  to  that  of  producing 
a  durable  carbon,  is  to  make  filaments  which  will  be  all  alike 
when  finished.  To  be  commercially  successful  a  lamp-maker 
must  be  able  to  make  his  filaments  within  a  very  little  all 
alike  in  every  particular,  that  is  in  dimensions  (length,  circum- 
ference, cross-section),  in  quality  of  surface,  and  in  specific 
resistance.  Many  different  methods  of  preparing  carbon  have 
been  proposed.  Whatever  the  method,  there  is  one  operation 
which  almost  all  of  them  necessarily  include — that  of  roasting 
or  baking  the  material  in  a  furnace  out  of  contact  with  the 
atmosphere,  in  order  to  drive  off,  as  far  as  possible,  every- 
thing contained  in  the  material  which  is  not  carbon.  In 
most  cases  an  entire  change  in  the  nature  of  the  material  is 
produced  by  the  process.  One  of  the  first  methods  to  be  tried 
was  that  of  forming  the  filament  out  of  a  paste  made  of 
ground  carbon,  with  a  binding  agent  such  as  a  solution  of 
sugar  or  caromel.  Such  a  paste  was  squirted  or  moulded  to 
the  desired  shape,  dried,  and  then  baked.  This,  of  course, 


THE    FILAMENT.  5 

produced  practically  the  same  material  as  is  used  for  the 
carbons  of  arc  lamps.  The  difficulty  in  this  method  is  to 
produce  fine  and  uniform  filaments.  In  any  event,  however, 
the  resulting  carbon  is  not  compact  enough  for  the  purpose, 
and  it  rapidly  disintegrates  when  used  in  a  lamp.  Edison 
proposed  a  mixture  of  lamp-black  and  tar,  an  utterly  worth- 
less process  in  itself,  but  one  which  has,  nevertheless,  become 
famous  in  the  history  of  lamp-making.  Processes  such  as 
these  involve  the  baking  of  the  material  in  order  to  drive  off 
the  volatile  matter  contained  in  the  binding  agent. 

The  successful  processes  are  not  those  by  which  the  fila- 
ments are  formed  direct  from  a  substance  containing  carbon 
as  such,  but  from  a  material  containing  carbon  in  chemical 
combination,  such  as  silk,  hair,  woody  fibre,  or  pure  cellulose. 
Such  substances  require  baking  at  a  high  temperature  in 
order  to  reduce  them  to  carbon,  their  other  constituents  being 
driven  off  by  the  heat.  The  process  of  baking  in  this  case  is 
generally  called  carbonisation.  The  only  processes  which  do 
not  include  that  of  baking  or  carbonisation  are  those  by 
which  the  filaments  are  deposited  or  built  up  from  the  carbon 
contained  in  some  vapour  or  liquid  such  as  illuminating  gas 
or  petroleum  oil. 


CHAPTER  II. 


PREPARATION  OF  THE  FILAMENT. 

SOME  of  the  methods  of  preparing  filaments  will  now  be 
described.  In  spite  of  all  the  work  that  has  been  done  by 
experimenters  during  the  last  ten  or  fifteen  years  with  a  view 
to  providing  something  new  and  better,  it  remains  a  fact  that 
the  original  process  of  Swan's,  that  of  parchmentising  cotton 
thread  by  sulphuric  acid,  remains  to  this  day  one  of  the  best, 
if  not  the  best  and  most  easily  worked  of  any.  It  will,  there- 
fore, be  described  first. 

The  raw  material  in  Swan's  process  is  pure  cotton,  such  as 
is  sold  for  knitting,  loosely  spun  into  a  thread.  It  is  usual, 
in  the  first  instance,  to  clean  the  thread  by  boiling  it  hi  soda 
or  ammonia,  in  order  to  take  out  any  grease  which  may  be 
present  in  it,  and  which  would  prevent  the  sulphuric  acid  from 
acting  on  those  parts  which  would  be  thereby  protected.  It 
must,  however,  be  thoroughly  washed  afterwards,  in  order  to 
remove  all  traces  of  the  alkali,  and  be  then  dried. 

It  is,  after  all,  an  open  question  if  the  above  process  of 
boiling  in  alkali  does  any  good,  provided  that  the  cotton  is  as 
clean  as  it  can  be  obtained  in  the  first  instance,  for  in  spite  of 
the  process,  there  are  often  found  to  be  places  in  the  thread 
upon  which  the  acid  will  not  act,  or  rather  upon  which  it  will 
not  act  in  the  tune  allowed.  Sometimes  these  places  are  very 
small,  and  can  only  be  seen  by  examining  the  parchmentised 
thread  very  closely,  while  at  other  times  they  will  extend  for 
several  inches  along  the  thread,  and  occur  at  short  intervals. 
No  satisfactory  explanation  of  these  acid-proof,  or  partially 
acid-proof,  sections  has  yet  been  given,  or,  at  any  rate,  no 
satisfactory  remedy  has  been  proposed. 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


PREPARATION    OF    THE    FILAMENT.  V 

When  dry,  the  cotton  thread  is  wound  on  a  drum,  and 
is  afterwards  unwound  and  drawn  through  a  dish  of  sul- 
phuric acid  of  the  required  strength,  whence  it  is  drawn  into 
water  and  wound  on  another  drum,  which  is  preferably  wholly 
under  the  water. 

This  sounds  a  very  simple  and  easy  process,  and  is, 
provided  that  certain  precautions  are  taken.  In  the  first 
place,  the  thread,  after  passing  through  the  acid,  is  extremely 
weak,  and  must,  accordingly,  be  drawn  along  as  regularly  and 
with  as  little  friction  as  possible.  To  insure  this,  the  drum 
upon  which  the  thread  is  wound  after  passing  through  the 
acid,  and  by  which  it  is  drawn  along,  must  be  driven  by 
power,  and  must  be  so  geared  as  to  run  very  steadily  and 
regularly  without  any  jerking.  The  slightest  friction  any- 
where will  put  a  strain  on  the  thread  and  will  cause  it 
•to  break. 

Fig.  1  shows  an  arrangement  which  the  Author  has  found 
to  work  very  well.  At  the  left-hand  end  is  seen  the  drum  A, 
upon  which  the  thread  is  wound  in  the  first  instance.  K  is 
the  drum  on  which  the  thread  is  ultimately  wound  and  which 
draws  it  along.  The  thread  has  to  be  guided  in  its  passage 
from  one  drum  to  the  other,  so  as  to  pass  for  the  right  length 
of  time  through  the  acid  and  then  into  the  water.  It  must 
be  remembered  that  the  action  of  the  acid  continues  from 
the  moment  when  the  thread  enters  the  acid  until  it  enters 
the  water,  though  the  thread  is  only  actually  under  the  acid 
in  the  dish  for  a  short  time,  after  which  it  passes  through  the 
air  until  it  reaches  the  water. 

In  order  to  guide  the  thread  with  the  minimum  of  friction, 
glass  rods  with  an  open  loop  formed  at  the  end  (Fig.  2),  may 
be  used,  the  thread  passing  through  the  loop.  The  advantage 
of  this  form  is  that  the  thread  may  be  put  into  the  loop  while 
in  motion  without  having  to  thread  the  end  through  first. 
A  number  of  blocks  (D,  E,  F,  G,  Fig.  1)  are  mounted  on  a 
bar  about  18in.  above  the  bench,  each  capable  of  sliding 
lengthwise  along  the  bar.  To  each  block  one  of  the  glass 
guide-rods  is  attached,  so  that  it  may  slide  up  or  down  and  be 
held  at  the  required  height  by  a  clamp.  Between  the  left- 
hand  drum  A  and  the  first  guide-rod  D,  a  sheet  of  glass,  B,  is 
laid  on  the  bench.  H  is  a  dish  containing  the  acid.  J  is  a 


10  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

lead-lined  trough  of  some  6ft.  in  length,  terminating  in  a 
tank  L,  which  contains  the  winding  drum  K,  driven  from 
overhead  by  a  band. 

In  starting  the  apparatus,  sufficient  thread  is  wound  off  the 
drum  A,  so  that  the  end  will  reach  to  K.  More  thread  is 
then  wound  off  A,  and  laid  zig-zag  fashion  across  the  glass- 
plate  B.  The  first  length  of  thread  is  then  dropped  into  the 
glass  guide-rods  and  the  end  is  looped  on  to  a  projection  on 
the  edge  of  the  drum  K  as  it  revolves.  The  thread  is,  there- 
fore, wound  on  to  K,  and  drawn  along  from  off  the  plate  B. 
One  of  the  guide-rods,  say  F,  is  now  lowered  so  that  it 
depresses  the  thread  below  the  surface  of  the  acid  in  the 
dish  H.  On  emerging  from  the  acid,  the  thread  passes  up 
through  the  guide  G,  and  then  drops  by  its  own  weight  into 
the  water  in  the  trough  J.  The  object  of  the  trough  J  is 


FIG.  2.— Glass  Guide  Rod  for  Thread. 

that  the  thread  may  be  examined  easily  in  order  to  see  if  it 
is  receiving  the  proper  amount  of  treatment.  It  can  be  best 
examined  while  passing  along  this  trough,  as  it  is  readily 
seen  in  the  water  against  the  lead  lining  of  the  trough.  On 
coming  out  of  the  acid  dish,  the  thread  has  a  transparent  and 
bright  appearance,  like  a  string  of  jelly.  When  it  enters  the 
water  it  is  immediately  changed  from  the  transparent  to  a 
semi-opaque  condition.  It  is  at  this  point,  after  entering  the 
water,  that  it  must  be  constantly  watched  to  see  if  it  is 
receiving  the  proper  amount  of  treatment.  If  it  is  not  being- 
done  sufficiently,  it  will  have  a  core  of  unconverted  cotton 
running  through  it  continuously  or  in  patches.  If  it  is 
receiving  the  proper  amount  of  treatment  it  will  appear  as 
a  semi-opaque  homogeneous  thread.  On  the  other  hand,  if 


PREPARATION    OF    THE    FILAMENT.  11 

the  treatment  is  too  long  the  thread  will  probably  break.  It 
is,  therefore,  necessary  at  starting  to  give  it  less  than  the 
proper  amount  of  treatment.  The  acid  dish  can  then  be 
moved  gradually  further  back,  so  that  the  thread  has  a  longer 
distance  to  travel  from  the  acid  to  the  water.  If  more  treat- 
ment than  can  be  given  by  this  means  is  required,  the  speed 
of  the  drum  K  must  be  reduced.  When  the  apparatus  is 
adjusted,  the  acid  dish,  the  edge  of  which  is  ground  level,  is 
covered  by  pieces  of  sheet-glass  so  that  the  acid  is  not  exposed 
to  the  air,  only  a  small  opening  being  left  just  where  the  thread 
enters  and  leaves.  As  the  thread  is  drawn  along,  it  is,  of 
course,  taken  up  off  the  glass  plate,  and  the  attendant  must 
take  care  to  have  more  thread  wound  off  the  drum  and  laid 
across  the  glass  plate  before  the  last  lot  is  used  up,  or  a 
breakage  will  occur.  Another  plan  is  to  drive  the  drum  A  as 
well  as  K  by  power.  The  thread  is  then  unwound  from  A 
as  it  is  wanted.  The  former  method,  however,  gives  less 
trouble.  By .  the  method  of  laying  out  the  thread  in  zigzag 
rows  on  the  glass  plate  the  attendant  has  sufficient  time  to 
examine  the  thread  as  it  passes  along  the  trough  and  to  keep 
the  apparatus  working  properly.  A  sheet  of  glass  is  required, 
because  the  fine  fibres  of  the  loosely- spun  thread  are  apt  to 
<jatch  if  laid  out  on  an  ordinary  wooden  bench,  unless  it  is  of 
hard  polished  wood.  The  slight  momentary  strain  produced 
in  this  way  is  often  sufficient  to  cause  a  breakage.  In  order 
to  still  further  reduce  the  strain,  porcelain  or  glass  pulleys 
driven  at  the  proper  speed  may  be  used  instead  of  the  glass 
guide-rods  (Fig.  2).  The  latter,  however,  are  much  simpler, 
and  answer  the  purpose  quite  well.  One  girl  can  easily  work 
the  apparatus.  There  is  a  supply  of  water  at  the  bottom  of 
the  tank  L,  with  an  overflow  at  the  end  of  the  trough  J. 
Fresh  water  is  thus  always  coming  in  by  the  drum,  and 
passing  along  the  trough.  The  strong  acid  carried  into  the 
water  by  the  thread  is  in  this  way  washed  out  without  getting 
far  along  the  trough. 

The  strength  of  the  acid  is  an  important  consideration.  A 
specific  gravity  of  1*64,  as  recommended  by  Mr.  Swinburne, 
works  very  well.  This  is  a  little  stronger  than  that  men- 
tioned by  Mr.  Swan  in  his  patent.  The  temperature  at  which 
it  is  used  should  be  from  60°  to  TOT.  A  large  quantity  of  acid 


12  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

should  be  made  up  to  the  required  strength  and  drawn  off 
in  small  quantities  as  required. 

It  is  a  curious  fact  that  the  finer  threads  are  more  easily 
worked  than  the  larger,  and  are  less  liable  to  breakage.  A 
much  shorter  immersion  in  the  acid  is  required,  as  a  rule, 
for  the  larger  sizes  of  thread  than  the  smaller.  The  speed 
at  which  the  thread  must  travel  is  from  12ft.  to  24ft.  per 
minute,  and  the  length  of  thread  under  acid  (from  where  it 
enters  the  acid  to  where  it  enters  the  water)  will  be  from 
12in.  to  86in.  The  actual  time,  therefore,  during  which  the 
acid  is  acting  on  the  thread  will  be  from  two  and  a-half  to 
fifteen  seconds,  according  to  the  quality  and  size  of  the  thread 
and  the  tightness  to  which  it  is  spun. 

Guide-rods  may  be  arranged  in  front  of  the  winding-drum 
K,  so  as  to  gradually  move  the  thread  along  the  length  of  the 
drum  and  ensure  it  winding  evenly.  This,  again,  is  an  un- 
necessary refinement.  A  glass  rod  fixed  to  a  piece  of  wood, 
which  can  be  laid  across  the  trough  and  moved  by  hand  every 
now  and  again,  is  quite  sufficient.  It  does  not  matter  if  the 
thread  is  wound  on  the  drum  in  several  layers,  as  it  is  easily 
wound  off  afterwards,  and  the  washing  of  several  layers  does 
not  take  much  longer  than  a  single  one.  If  only  one  layer  is 
wound  on  the  drum,  the  number  of  drums  and  washing  tanks 
is  multiplied,  and  time  is  lost  in  changing  the  drums  more 
frequently.  The  drum  K  may  be  made  with  a  wood  spindle 
and  ends,  and  covered  with  copper  gauze,  which  allows  of  a 
more  rapid  and  thorough  washing  of  the  thread  than  a  solid 
surface  would  do.  When  a  drum  is  filled  it  is  lifted  out  of  the 
tank  L  and  removed  to  another  tank,  in  which  the  thread  is- 
further  washed  by  a  continual  circulation  of  water.  In  twelve 
hours  or  so  all  traces  of  acid  will  have  been  removed  from  the 
thread,  which  is  then  ready  for  drying. 

The  parchmentised  thread  quickly  dries  on  exposure  to  dry 
air,  and  in  doing  so  it  hardens  and  contracts.  Conse- 
quently, in  order  that  it  may  dry  evenly  and  straight,  it  is 
necessary  to  take  it  off  the  drums  on  which  it  has  been  washed 
and  hang  it  up.  A  convenient  method  of  drying  is  shown 
in  the  accompanying  sketch,  Fig.  8. 

A  number  of  bars  of  wood  are  fixed  parallel  to  each  other,. 
8ft.  or  Oft.  above  the  floor  of  the  drying-room  and  about 


PREPARATION  OF  THE  FILAMENT. 


13 


2ft.  apart.  On  each  side  of  these  bars,  B,  are  secured  a 
number  of  porcelain  insulators,  A  A,  at  intervals  of  a  little 
less  than  their  own  diameter,  about  2in.  Immediately  below 
the  bars  is  fixed  on  the  floor  a  framework  consisting  of 
three  boards  set  up  edgewise,  the  two  outer  ones  not  reaching 
to  the  floor  by  2in.  or  so,  except  at  the  ends,  the  space 
between  the  two  outer  boards  and  the  centre  one  being 
a  little  more  than  the  width  of  the  insulators.  In  order  to 
dry  the  parchmentised  thread,  the  drum  on  which  it  is  wound 
is  taken  out  of  the  washing  tank  and  set  in  a  stand  so  that  it 


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FIG.  3.— Method  of  Drying  Parchmentised  Thread. 

can  be  turned  round.  One  operative  takes  the  end  of  the 
thread  and  ties  a  loop  on  it,  and  standing  on  a  pair  of  steps, 
hangs  the  loop  on  the  farthest  insulator.  A  second  girl 
unwinds  the  drum,  and  a  third  one  takes  the  thread  as  it 
comes  off  and  hands  it  to  the  first  one,  who  hangs  it  again 
over  the  second  insulator,  while  she  (No.  2)  proceeds  to  hang 
another  insulator  in  the  loop  thus  formed  and  lower  it  between 
the  boards,  the  length  of  the  loop  being  so  adjusted  that  the 
insulator  hangs  at  the  bottom  of  the  outside  board.  This 
operation  is  continued  until  the  whole  length  of  the  parch- 
mentised thread  is  hung  up,  with  a  weight  on  every  loop  to 


14  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

keep  it  stretched.  As  the  thread  dries  it  contracts  and  raises 
the  weights  hanging  in  the  loops  to  nearly  the  top  of  the 
boards.  When  dry  the  thread  has  become  again  more  trans- 
parent and  has  somewhat  the  appearance  of  catgut.  It  is 
then  taken  down  and  wound  on  a  reel,  and  is  ready  for 
the  next  process  of  cutting  to  size.  The  boards,  C  C  C,  are 
for  the  purpose  of  preventing  the  weights  from  falling  off 
the  thread  and  the  loop  from  twisting  on  itself.  When 
taking  the  thread  down  the  weights  are  easily  shaken  off. 
The  outside  boards,  C  C,  are  fixed  above  the  ground  so  that 
the  weights  can  be  got  out  easily  if  they  fall  between  the 
boards.  This  method  of  drying  insures  that  the  thread  all 
along  gets  the  same  tension.  For  thick  thread  it  may  be 
necessary  to  use  heavier  weights  than  for  the  smaller  sizes. 

Another  method  is  to  have  porcelain  pulleys  instead  of  the 
insulators,  and  the  bottom  ones  fixed  as  well  as  the  top.  The 
thread  is  then  wound  up  and  down,  and  one  weight  only  hung 
upon  the  end.  This  method,  however,  puts  on  a  varying 
strain  owing  to  the  friction  in  the  pulleys,  and  some  lengths 
will  be  pulled  tighter  than  the  others.  Besides,  the  thread, 
as  soon  as  it  begins  to  get  dry,  stiffens,  and  wih1  not  readily 
pass  over  the  pulleys.  In  the  first  method  it  will  be  seen 
that  the  thread  is  not  required  to  move  past  the  pulleys  as  it 
contracts. 

The  chemical  action  of  this  parchmentising  process  is 
remarkable,  inasmuch  as  the  parchmentised  thread  appears  to 
retain  the  same  composition  as  the  thread  (C6H1005)  from 
which  it  is  made,  though  in  appearance  it  is  an  absolutely 
different  substance.  There  is,  however,  an  increase  in  weight 
equal  to  about  8  per  cent.  The  substance  so  produced  is 
called  amyloid,  and,  as  will  presently  be  explained,  it  can  be 
produced  by  other  methods.  The  amyloid  thread  produced  as 
shown  has  a  rough  and  uneven  surface,  and  it  must  be  shaved 
down  and  made  even  before  it  is  fit  for  use.  This  cutting 
down  is  accomplished  by  pulling  the  parchmentised  thread 
through  sharp-edged  draw-plates.  As  there  are  often  lumps 
on  the  thread,  it  has  to  be  drawn  through  several  sizes  of 
draw-plates  in  order  to  remove  these  lumps  before  it  is  passed 
through  the  smaller  sizes,  which  will  shave  it  along  its  whole 
length.  If  it  is  drawn  through  too  small  a  plate  at  first  it 


I'KKl'AUATIOX    OF    THE    FILAMENT. 


15 


•will  break  when  the  thick  parts  reach  the  draw-plate.  Steel 
draw-plates  can  be  used,  but  the  cutting  edge  will  very  soon 
become  blunt.  Jewelled  plates  are  therefore  generally  used, 
and  even  the  hard  stones  used  in  these  frequently  require 
sharpening.  The  size  of  thread  parchnientised  is  proportioned 
as  nearly  as  possible  to  the  size  necessary  for  the  particular 
filaments  required,  so  that  there  shall  be  no  more  cutting 
down  than  is  necessary  for  obtaining  even  filaments. 


FIG.  4. — Circular  Jewelled  Draw-Plate  for  Cutting  Parchmentised  Thread. 

The  draw-plates  used  for  the  larger  sizes  of  thread  may 
be  similar  to  the  ordinary  jewelled  wire  draw-plates,  but 
with  a  sharp-cutting  edge  (Fig.  4).  For  the  smaller  sizes, 
however,  below  about  twenty  mils,  it  is  necessary  to  use 
split  draw-plates,  owing  to  the  difficulty  of  threading  them. 


FIG.  5. — Split  Jewelled  Draw-Plate  for  Cutting  Parchmentised  Thread. 

Fig.  5  is  an  illustration  of  a  split  draw-plate.  The  ends  of 
the  larger  sizes  of  thread  can  be  easily  pointed  with  a  knife, 
be  pushed  through  the  circular  draw-plates,  and  be  pulled 
through  at  first  with  tweezers.  It  is,  however,  very  difficult 


16  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

to  do  this  with  the  smaller  sizes,  and  it  is,  therefore,  better 
to  use  the  split  form.  The  circular  form  is  used  whenever 
possible,  as  it  costs  very  much  less  than  the  split  one.  The 
thread  must  be  drawn  through  the  plates  as  regularly  as 
possible,  and  for  this  reason  the  plate  is  fixed  upright  on 
a  bench,  and  12in.  or  so  behind  it  there  is  a  wooden  drum 
turned  by  power.  A  couple  of  feet  or  so  of  the  thread 
is  pulled  through  the  die  by  hand  and  then  turned  once  or 
twice  round  the  drum,  which  then  does  all  the  pulling  in  a 
steady  and  even  manner,  so  that  the  thread  is  much  less  likely 
to  break  than  if  it  is  drawn  by  hand.  A  guide  should  be 
fixed  both  in  front  and  behind  the  draw-plate,  and  in  line 
with  the  hole,  so  that  the  thread  is  always  drawn  through 
perfectly  at  right  angles  to  the  front  surface  which  contains 
the  cutting  edge.  If  the  thread  is  not  drawn  through  straight 
it  will  be  cut  unevenly  and  of  an  oval  section  instead  of 
•circular.  When  the  jewel  becomes  dull,  and  will  no  longer 
cut  properly,  it  can  be  sharpened  and  made  as  good  as  new 
again,  though  it  will  by  the  process  be  made  larger  in  the 
hole,  and  will  only  serve  for  a  larger  size  of  thread  than  before. 
The  thread  must  be  passed  through  several  sizes  of  plate,  each 
a  little  smaller  than  the  last.  The  amount  cut  off  by  each  plate 
should  not  be  more  than  about  5  per  cent,  of  the  diameter  of 
the  thread,  except,  perhaps,  at  starting,  when  lumps  only  are 
being  cut  down.  This  process  of  cutting  down  when  properly 
carried  out  gives  the  thread  a  smooth,  polished  surface,  and 
makes  it  perfectly  circular.  It  is  then  ready  for  being  blocked 
for  carbonisation. 

There  is  another  process  of  treating  cotton  so  as  to  produce 
a  substance  practically  identical  with  Swan's  parchmentised 
•cotton  thread,  which  possesses  several  great  advantages  over 
that  process.  It  consists  in  dissolving  cotton  wool  in  a  solu- 
tion of  chloride  of  zinc.  The  viscous  solution  produced  by 
this  means  is  squirted  through  a  small  hole  into  a  vessel  con- 
taining alcohol  which  causes  it  to  set  and  harden.  The  great 
advantage  of  this  method  is  that  it  produces  a  thread  of  amy- 
loid of  uniform  diameter  throughout,  and  which  can  be  made 
of  any  size,  according  to  the  size  of  the  hole  through  which 
it  is  squirted,  and  which  requires  no  after  process  of  cutting 
clown  by  means  of  draw-plates.  This  is  a  very  great  advantage 


PREPARATION    OF   THE    FILAMENT.  17 

as  the  wear  and  tear  of  the  draw-plates  is  considerable  and 
the  cost  of  the  process  is  saved.  This  process,  like  all  others, 
requires  great  care  if  uniform  results  are  to  be  achieved. 
The  strength  of  the  zinc  chloride  solution  and  the  amount  of 
cotton  dissolved  in  it  must  be  carefully  regulated,  as  also  must 
the  temperature  at  which  it  is  kept  while  the  cotton  is  dissolv- 
ing. There  are  several  difficulties  met  with.  The  cotton  wool 
when  put  into  the  solution  carries  down  with  it  a  quantity  of 
air,  which,  as  the  cotton  dissolves,  is  seen  in  the  form  of 
bubbles  all  through  the  solution.  The  solution  produced  is  of 
such  a  viscous  nature  that  if  left  to  itself  these  bubbles  will 
remain  wherever  they  happen  to  be,  or  at  any  rate  they  will 
rise  so  slowly  that  it  would  be  a  matter  of  weeks  for  them  to 
reach  the  surface.  In  order  to  get  rid  of  them  the  solution  is 
filtered  and  at  the  same  time  heated  under  a  vacuum.  In 
this  way  most  of  them  may  be  extracted.  If  they  are 
not  got  rid  of  before  the  solution  is  squirted,  the  very  small 
ones  will  remain  in  the  squirted  thread,  while  those  which  are 
of  a  size  equal  to,  or  larger  than,  the  diameter  of  the  nozzle  of 
the  squirt,  will  cause  a  break  in  the  thread.  The  smallest  air 
bubbles  are  the  most  troublesome,  as  their  presence  may  not 
be  detected  until  the  filament  comes  to  be  lighted  up,  when  a 
bright  spot  will  appear  at  the  place  where  the  bubble  was 
located.  It  is  also  very  important  that  there  should  be  no 
lumps,  or  parts  of  the  solution  thicker  than  the  rest.  When 
these  occur,  and  arrive  at  the  nozzle  of  the  squirt,  they 
may  retard  or  stop  the  flow,  and  greater  pressure  has  to  be 
put  upon  the  solution  to  force  it  through.  As  soon  as  the 
lump  is  through,  the  thinner  portion  of  the  solution  following 
it  will  run  too  quickly.  The  result  is  that  the  thread  is  not 
uniform  in  diameter.  Constant  stirring  during  the  dissolving 
will  prevent  the  formation  of  lumps  to  a  great  extent.  Messrs. 
Wynne  and  Powell,  who  patented  this  process  in  1884,  give 
the  specific  gravity  of  the  zinc  solution  required  as  1-8  and  the 
temperature  100°C.  The  material  finally  requires  thorough 
washing  in  alcohol  or  water.  The  drying  may  be  done  in  the 
same  way  as  already  described  for  Swan's  process. 

Cotton  is  again  the  raw  material  of  another  process  for 
making  filaments  —  the  process  invented  by  Mr.  Weston. 
Cotton  is  converted  by  the  well-known  process  into  pyroxyline, 

o 


18  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

and  then  into  the  material  known  as  celluloid.  This  is  rolled 
into  thin  sheets,  which  are  then  treated  with  ammonium 
sulphide,  which  reduces  them  again  to  the  chemical  state  of 
cellulose,  though  leaving  them  transparent  and  flexible.  The 
sheets  are  then  treated  with  bisulphide  of  carbon  or  turpentine, 
in  order  to  remove  all  traces  of  sulphur,  and  are  afterwards 
either  cut  up  into  narrow  strips  or  pieces  are  punched  out 
of  them  of  the  required  size  and  shape.  The  strips  may  be 
made  into  spirals  or  other  forms  by  winding  them  on  man- 
drils and  heating  them  to  a  temperature  which  will  stiffen 
them,  so  that  they  will  retain  their  shape  when  removed  from 
the  mandrils.  The  sheets  of  celluloid  may  be  cut  into  strips, 
or  punched  out,  and  the  ammonium  sulphide  treatment  applied 
afterwards.  Carbons  made  by  this  process  have  a  lustrous, 
highly-polished,  metallic  appearance. 

A  process  producing  very  similar  filaments  to  the  above  is 
that  which  uses  a  peculiar  liquid  hydrocarbon  called  furfurol 
as  a  basis.  This  liquid  is  treated  with  sulphuric  acid,  and  the 
black  liquor  produced  is  poured  upon  a  sheet  of  glass.  A 
second  sheet  of  glass  is  then  placed  upon  the  top.  The  space 
between  the  sheets  is  determined  by  a  strip  of  paper  placed 
along  the  margin.  In  about  twelve  hours  the  liquor  will  have 
set,  and  the  glass  plates  are  forced  apart.  The  sheet  of  black 
material  produced  adheres  to  one  of  the  glass  plates,  from 
which,  however,  it  can  be  easily  detached  under  water.  The 
sheet  is  then  cut  up  into  strips,  which  can  be  moulded  to 
shape  in  the  same  way  as  in  the  Weston  process,  by  heating, 
and  are  then  ready  for  carbonising.  This  process,  again,  may 
be  considered  as  starting  from  cotton,  as  the  liquid  furfurol 
is  obtained  by  the  distillation  of  cotton  or  other  form  of 
cellulose.  The  Author  has,  however,  no  personal  experience 
in  working  this  process. 

Cotton  was,  again,  the  basis  in  a  process  proposed  by 
Prof.  Crookes.  Cotton  was  dissolved  in  a  solution  of  cupram- 
monia,  and  sheets  of  the  substance  were  formed  on  glass,  as 
in  the  last  process.  The  Crookes'  process  has,  however, 
not  been  extensively  used.  The  Author  has  tried  squirting 
filaments  of  this  preparation,  but  owing  to  the  very  great 
shrinkage  of  the  squirted  thread  when  drying,  the  method 
was  found  to  be  useless. 


PREPARATION   OF    THE    FILAMENT.  19 

Parchmentised  paper  was  proposed  by  Swan  before  his 
cotton  process.  Ordinary  hard  paper,  without  any  chemical 
treatment,  has  also  been  used.  In  all  of  the  above  processes 
the  carbons  are  produced  by  the  carbonisation  of  cellulose  or 
some  modification  of  it.  All  vegetable  fibres  contain  cellulose, 
cotton  being  nearly  pure  cellulose,  and  many  of  them  can 
be  carbonised  and  made  to  serve  for  lamp  filaments  either 
with  or  without  any  chemical  treatment.  Bass  fibre,  tampico 
fibre,  and  many  kinds  of  grasses  have  been  tried.  Some  of 
them  make  very  good  lamps,  but  the  great  objection  to  all 
is  the  want  of  uniformity  in  size  and  composition.  It  is  not 
likely  that  any  of  them  will  survive  as  commercial  lamp 
filaments  for  this  reason.  Edison,  as  is  well  known,  used 
bamboo,  and  with  the  aid  of  very  ingenious  machinery 
stamped  out  pieces  of  the  required  size.  Specimens  of  the 
bamboo  in  the  various  stages  of  reducing  to  size  were  ex- 
hibited at  the  Crystal  Palace  Exhibition  in  1882,  and  will  be 
found  described  and  illustrated  in  Dredge's  "  Electric  Illu- 
mination." 

Many  other  substances  have  been  tried.  Of  animal  sub- 
stances silk  is  undoubtedly  the  most  successful,  and  may 
be  carbonised  without  any  previous  chemical  treatment.  All 
animal  substances,  however,  require  special  precautions  in  the 
matter  of  temperature  during  carbonisation. 


c  2 


CHAPTER  III. 


CARBONISATION. 

THE  process  of  carbonisation  consists  in  baking  the  car- 
bonisable  material  at  a  very  high  temperature  out  of  contact 
with  the  air.  During  the  process  the  material  will  shrink  and 
become  hard  and  stiff,  and  will  take  permanently  whatever 
shape  it  happens  to  be  in  at  the  time.  Special  arrangements 
have,  therefore,  to  be  made  to  control  the  shape  of  the  fila- 
ments. The  methods  adopted  for  accomplishing  this  will  now 
be  described. 

First  of  all  in  the  case  of  long  threads  as  produced  by 
Swan's  process. 

For  making  plain  horse -shoe  filaments  the  thread  is  wound 
on  carbon  blocks  or  forms.  Arrangements  must,  however,  be 
made  for  the  shrinkage.  The  material  will  shrink  both  in 
length  and  thickness,  becoming  about  one-third  smaller.  If 
wound  on  a  solid  carbon  block  and  then  carbonised  the 
thread  will  be  broken  in  every  turn  or  loop.  Various 
methods  of  allowing  for  the  shrinkage  have  been  used.  The 
one  shown  in  the  illustration  (Fig.  6)  is  a  good  one. 

A  is  the  carbon  block,  with  two  holes  about  a  quarter  of 
an  inch  diameter  drilled  in  it.  B  is  a  circular  piece  of 
carbon,  the  diameter  of  which  is  the  same  as  the  thickness  of 
the  block  A.  There  are  two  holes  drilled  into  B  correspond- 
ing to  those  in  A.  At  the  top  A  should  be  well  and  smoothly 
rounded.  A  and  B  are  connected  together  by  sticks  of  hard 
wood  C  C,  which  may  fit  tightly  in  the  holes  into  which  they 
are  driven  home,  reaching,  therefore,  to  within  half  an  inch 
or  so  of  the  top  of  the  block.  The  thread  is  then  wound  round 
the  block  and  end  piece.  It  may  either  be  wound  on  by  hand, 


22 


THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


or  the  block  may  be  clamped  in  a  machine  which  will  turn 
it  round  and  wind  it  evenly,  the  machine  feeding  the  block 
forward  at  the  same  time.  When  the  block  is  filled  it  should 
be  wrapped  up  in  thin  calico  or  paper,  and  it  is  then  ready 
for  packing  in  the  crucible.  By  using  the  wooden  sticks 
C  C  instead  of  a  solid  carbon  block,  the  shrinkage  of  the  thread 
is  allowed  for.  The  block  is  placed  in  the  crucible  with  A 
uppermost,  care  being,  taken  that  it  stands  loosely  and  is  not 


r\ 


FIG.  6. — Carbon  Frame  for  Carbonisation  of  Single-Looped  Filaments. 

packed  too  tightly.  As  the  process  of  carbonisation  proceeds, 
the  wood  C  C  shrinks,  and  the  block  A  descends  at  the  same 
time  that  the  thread  contracts.  By  carefully  regulating  the 
temperature  the  thread  is  thus  allowed  to  contract  without 
being  strained  sufficiently  to  break  it,  but  just  enough  to  keep 
it  perfectly  straight.  The  length  of  the  sticks  C  C  is  such 
that  when  the  carbonisation  is  complete  the  block  A  will  have 
sunk  upon  the  end-piece  B,  the  sticks  being  entirely  enclosed 
in  the  holes.  The  illustration  shows  the  block  A  and  the 


CARBONISATION. 


23 


end-piece  B  in  position  for  winding  on  the  thread.     After 
carbonisation  A  and  B  are  close  together. 

If  it  is  desired  to  get  a  long  filament  into  a  comparatively 
short  bulb,  the  filament  may  be  bent  round  into  a  loop,  as  in 
the  Swan  lamp.  This  may  be  accomplished  by  simply  using 
two  of  the  round  end  pieces  of  the  carbon  blocks  already 
described,  joined  together  by  the  wooden  sticks.  The  thread 
is  then  wound  with  a  complete  turn  round  the  carbon  at  each 
end,  the  wooden  sticks  being  long  enough  to  allow  of  two  fila- 
ments end  to  end.  Instead  of  winding  the  thread  straight 
down  from  one  carbon  to  the  other  like  the  figure  0,  it  may 
be  crossed  like  an  8.  The  use  of  solid  carbon,  however,  is  not 
good,  owing  to  insufficient  allowance  for  contraction  in  the 


of!    HE     A       E^l 
U     U  JJ    ! 7 

~ B~~  TTfK  J 


H 

1 


iJ    U 


FIG.  7. — Carbon  Frame  for  Carbonisation  of  Double-Looped  Filaments. 

loop.  The  thread  will  draw  round  the  carbon  in  some  degree 
as  it  shrinks,  but  only  to  a  small  extent,  the  result  being  that 
the  filament  is  strained  in  the  loop.  This  may  be  overcome 
by  using  split  carbons  instead  of  solid  ones  to  wind  the  loop 
upon,  as  in  Fig.  7. 

A  and  B  are  the  semi-circular  carbons.  C  C  the  wooden 
sticks,  which  are  smaller  (D  D)  where  they  pass  through  the 
holes  in  the  carbons.  Each  piece  A  B  has  two  other  holes 
E  E,  into  which  are  inserted  short  wooden  sticks.  In  this 
way  allowance  is  made  for  the  shrinkage  in  the  loop  of  the 
filaments  as  well  as  in  the  long  sides. 

In  every  process  of  lamp  manufacture  it  is  necessary  to  use 
the  utmost  care  to  prevent  waste.  Very  slight  differences  hi 


24  THE    INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 

the  ways  of  carrying  out  some  of  the  processes  may  make  all 
the  difference  in  the  result.  Carbonisation  is  one  such  process. 
If  the  filaments  are  properly  packed  and  handled,  and  the  heat 
of  the  furnace  is  properly  regulated,  filaments  made  in  the 
above  manner  may  be  obtained  almost  without  loss,  certainly 
with  a  breakage  of  not  more  than  2  or  3  per  cent. 

It  is  not  sufficient,  when  the  thread  is  wound  on  blocks  in 
the  way  above  described,  to  pack  the  blocks,  whether  wrapped 
up  or  not  in  calico  or  paper,  into  a  crucible  and  fill  up  all  the 
remaining  space  with  powdered  carbon.  The  carbon  powder 
will  prevent  the  top  part  of  the  block  from  sinking  during  the 
shrinkage,  and  the  result  will  be  the  breaking  of  most  of  the 
filaments.  The  blocks  must  be  perfectly  free  to  move  as  the 
shrinkage  proceeds.  The  best  way  to  insure  this  result  is  to 
use  a  carbon  box  inside  the  crucible.  The  box  should  be  half 
an  inch  or  so  deeper  inside  than  the  length  of  the  blocks  when 
wound.  A  box  should  accommodate  about  ten  blocks.  The 
blocks  are  put  in  with  the  part  A  uppermost.  Between  each 
block  is  a  thin  plate  of  carbon  about  one-eighth  of  an  inch 
thick.  These  plates  slide  easily  in  grooves  in  the  sides  of  the 
box.  By  this  means  each  block  is  in  a  separate  chamber,  and 
is  entirely  free  to  move  during  the  shrinkage.  A  carbon  lid 
fits  loosely  on  the  top  of  the  box.  The  whole  carbon  box  is 
then  put  into  a  "  plumbago  "  crucible  of  the  same  shape,  but 
having  a  clearance  of  half  an  inch  or  so  all  round,  and  an  inch 
higher  than  the  lid  of  the  box.  The  space  between  the  box 
and  the  crucible  is  then  filled  up  with  powdered  charcoal  to 
the  level  of  the  top  of  the  crucible,  and  the  crucible  lid  is  then 
put  on  and  plastered  down  with  fire-clay,  care  being  taken  to 
leave  one  or  two  small  gaps  in  the  fire-clay  for  the  escape  of 
the  gases.  If  this  is  not  done,  the  fire-clay  may  hold  the  lid 
so  tightly  that  the  gases  cannot  escape  until  a  great  pressure 
has  been  produced  in  the  crucible,  when  the  lid  may  be  thrown 
violently  off  and  the  contents  spoiled.  No  powdered  charcoal 
is  required  in  the  carbon  box,  and  the  filaments  cannot  be 
burnt  at  all  by  the  small  quantity  of  air  left  in  the  box. 
Before  the  temperature  of  red  heat  which  would  be  required 
to  burn  the  filaments  is  reached,  most  of  the  enclosed  air  has 
been  driven  out  of  the  crucible  by  expansion.  The  gases 
produced  by  the  charring  of  the  filaments,  the  wood  sticks  and 


CARBONISATION.  25 

the  calico  wrapping  have  been  coming  off  in  such  volume  as 
to  sweep  out  practically  all  the  remaining  air,  so  that  by  the 
time  the  filaments  are  hot  enough  to  burn  there  is  no  air 
left  in  the  box  to  burn  them. 

Flat  filaments,  and  those  which  are  shaped  before  carboni- 
sation, of  course,  require  a  different  method  of  packing  in  the 
crucibles.  Spirals  and  such  like  forms  must  be  packed  in 
carbon  powder  or  plumbago  as  loosely  as  possible,  so  that 
they  may  retain  their  shape  during  the  shrinkage.  Such  a 
method,  however,  is  very  costly,  owing  to  the  comparatively 
small  number  of  filaments  which  can  be  accommodated  in  the 
•crucible.  Flat  filaments  may  be  packed  between  layers  of 
paper  or  calico  with  a  carbon  block  for  a  weight  on  the  top. 
If  calico  is  used,  it  should  be  partially  charred  before  using, 
as  it  shrinks  during  carbonisation  more  then  most  filament 
substances. 

Filaments  of  different  material  require  different  treat- 
ment during  carbonisation.  Some  materials  will  allow 
of  being  heated  much  more  rapidly  than  others,  and  some 
require  a  greater  and  more  prolonged  heat  than  others. 
Generally  speaking,  however,  it  may  be  said  that  the  tempera- 
ture should  be  raised  very  gradually  and  evenly,  and  should  be 
raised  as  high  as  possible. 

The  construction  of  the  furnace  is  also  an  important  con- 
sideration. The  fire  must  be  in  a  separate  chamber  from  the 
Crucibles,  which  must  not  be  liable  to  be  knocked  about  when 
the  fire  is  being  stoked.  It  is  well  if  the  fire  can  be  made  to 
reach  the  crucibles  evenly  all  round.  This,  however,  is  not 
important,  provided  that  the  temperature  is  at  no  time  suddenly 
raised. 

The  illustration,  Fig.  8,  shows  a  form  of  furnace  which  works 
very  well.  The  fire  is  in  a  chamber  on  the  left.  The  hot 
gases  pass  over  the  "  bridge,"  over  and  between  the  crucibles, 
and  down  through  a  flue  into  the  chimney  shaft. 

A  is  the  fire,  B  is  a  "  bridge  "  of  fire  bricks,  C  is  the  floor 
of  the  chamber,  D  is  the  flue  leading  to  the  chimney  shaft, 
E  E  are  the  crucibles,  F  is  a  damper  in  the  flue  D.  The  furnace 
door  is  made  of  fire-clay,  and  is  bound  together  with  iron.  It 
can  be  raised  and  lowered  by  means  of  a  chain  and  counter- 
weight running  over  a  pulley.  The  fire  door  can  also  be 


26  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

arranged  in  the  same  way.  The  furnace  door  has  a  small  peep- 
hole in  the  centre,  into  which  is  fitted  a  fire-clay  stopper.  This 
hole  also  serves  for  the  insertion  of  the  pyrometer  G,  which^is 
required  during  the  first  part  of  the  process.  The  furnace  must 
be  substantially  built  with  the  best  fire-clay  bricks,  and  must 
be  held  together  by  iron  tie  rods  and  bands.  The  crucibles 
must  be  of  the  plumbago  type,  and  of  the  best  quality.  The 


FIG.  8. — Carbonising  Furnace. 


temperature  they  have  to  withstand  is  so  great  that  they  will 
only  survive  a  few  bakings. 

The  part  of  the  process  of  carbonising  which  requires  most 
attention  is  the  first,  that  is  to  say,  before  the  temperature  of 
red  heat  is  reached.  If  a  crucible  containing  filaments  wound 
on  blocks  and  packed  as  described  is  put  at  once  into  a  hot 
furnace,  or  if  the  temperature  is  raised  too  suddenly,  the  result 
will  probably  be  that  all  the  filaments  are  broken.  If  quickly 


CARBONISATION. 


27 


heated,  the  filaments  on  the  blocks  begin  to  contract  before 
the  wooden  sticks,  which  take  a  longer  time  heating,  owing 
to  their  greater  thickness  and  the  quantity 'of  gases  which 
they  give  off.  These  gases,  being  very  rich  in  hydrocarbons 
(being,  of  course,  the  ordinary  products  of  destructive  distilla- 
tion of  wood),  deposit  a  thick  coating  of  mess  on  the  filaments 
and  on  the  carbon  block.  As  the  temperature  rises  this  mess  is 
further  decomposed,  leaving  behind  a  residue  of  carbon,  which 
firmly  cements  the  filaments  together  and  to  the  carbon  block, 
thus  spoiling  them  all.  When,  however,  the  temperature  is 
slowly  and  gradually  raised,  the  contraction  of  the  filaments 
and  the  sticks  takes  place  at  the  same  time,  and  the  gases 
which  are  given  off  from  the  sticks  pass  out  without  leaving  any 
deposit  whatever.  Gases  from  the  wood  continue  to  be  given 
off  up  to  red  heat,  as  may  be  seen  by  a  flame  at  the  small  holes 
left  in  the  luting  of  the  crucible  lid.  The  parchmentised  thread 


FIG.  9. — Pyrometer  for  Indie  iting  Temperature  during  Early  Stages  of 
Carbonisation. 

itself,  having  the  composition  C6H1005,  is  supposed  to  give  off 
only  steam  or  its  equivalent. 

It  will  thus  be  readily  understood  that  some  kind  of 
pyrometer  must  be  used,  during  at  any  rate  the  first  part  of 
the  heating,  up  to  nearly  red  heat.  Fig.  9  gives  an  illustra- 
tion of  a  suitable  instrument,  which  is  graduated  up  to 
600° C.,  or  to  just  dull  red  heat.  The  instrument  works  by 
indicating  on  the  dial  the  difference  in  expansion  of  rods  or 
tubes  of  different  metals  contained  within  the  outer  tube,  and  is 
sufficiently  good  for  the  purpose,  actual  temperatures  not  being 
so  important  as  the  rate  of  rise  of  temperature.  The  tube 
of  the  instrument  is  inserted  through  the  hole  in  the  furnace 
door,  a  support  on  the  back  wall  of  the  furnace  being  fixed  in 
the  right  position  for  the  end  to  rest  upon.  The  position  of 
of  the  tube  may  be  just  above  the  crucibles,  or  between  two 
rows  of  them. 


28  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

The  crucibles  being  placed  in  position  in  the  furnace,  the 
door  is  lowered  into  its  position,  and  cemented  round  the 
edge  with  fire-clay.  The  pyrometer  is  then  inserted  through 
the  hole  and  is  also  cemented  round  where  it  passes 
through  the  door.  The  fire  is  then  lighted,  and  by  means 
of  the  damper  is  kept  very  low  at  first ;  eight  hours  at  least 
should  be  allowed  for  traversing  the  range  of  temperature 
covered  by  the  pyrometer.  The  first  six  or  seven  hours  of 
this  time  may  be  in  daylight,  but  by  the  time  that  the  highest 
reading  on  the  pyrometer  is  reached  it  should  be  dark,  as  the 
regulation  from  this  point  will  have  to  be  done  by  aid  of  the 
eye.  The  eye  is,  of  course,  much  better  able  to  judge  of  colour 
(temperature)  after  dark  than  in  daylight,  when,  indeed,  the 
first  dull  red  heat  could  not  be  distinguished  at  all.  The 
highest  temperature  indicated  by  the  pyrometer  having  been 
reached,  the  instrument  is  withdrawn  and  the  peep-hole 
stopped  with  the  fire-clay  plug.  It  will  be  necessary  from 
this  time  to  watch  the  temperature  through  this  hole,  the 
plug  being  withdrawn  from  time  to  time  for  the  purpose. 
Another  five  or  six  hours  should  be  occupied  in  bringing  the 
temperature  gradually  up  to  a  bright  red  heat,  and  when  this 
is  reached  the  full  power  of  the  furnace  may  be  applied,  and 
three  or  four  hours  more  will  be  occupied  in  bringing  the  tem- 
perature up  to  a  white  heat.  The  temperature  which  can  be 
attained  by  a  furnace  of  this  kind  is  so  great  that  the  best  fire- 
bricks will  begin  to  run,  and,  having  raised  the  temperature 
to  the  highest  point,  there  is  probably  not  much  advantage 
gained  in  maintaining  it  for  any  length  of  time.  The  fire 
may,  therefore,  be  allowed  to  die  out  and  the  furnace  to  cool 
down.  This  again  must  be  done  gradually,  not  on  account  of 
the  danger  to  the  filaments,  but  to  the  furnace.  The  furnace - 
door  and  the  fire-door  must  be  kept  closed.  Eapid  cooling 
will  have  a  very  deleterious  effect  on  the  furnace,  which  will 
require  re-building  much  sooner  than  it  otherwise  would  do. 
Twelve  hours  at  least  should  be  allowed  for  cooling.  The 
crucibles  should  not  be  disturbed  until  they  are  cool  enough  to 
be  taken  hold  of  by  the  hand.  If  they  are  handled  while  too 
hot,  they  are  likely  to  be  bumped  about,  and  the  filaments  will 
be  damaged.  The  wear  and  tear  on  a  furnace  which  is  used  in 
this  way,  alternately  heated  and  cooled,  is  much  greater  than 


CARBONISATION.  29 

x 

that  on  a  furnace,  such  as  a  glass  furnace,  which  is  kept  con- 
stantly going  at  one  temperature.  If  heated  and  cooled  off 
every  two  days,  it  will  require  re -building  probably  every  six 
months.  When  the  crucibles  are  cool  they  are  withdrawn 
and  placed  on  a  truck  and  wheeled  to  the  room  where  they 
are  to  be  unpacked. 

With  a  furnace  of  this  kind,  where  the  fire  is  all  on  one  side 
of  the  crucibles,  it  might  be  expected  that  there  would  be 
trouble  owing  to  uneven  heating.  There  is,  however,  no 
trouble  when  the  temperature  is  raised  slowly,  as  it  should  be. 
The  powdered  charcoal  used  as  packing  and  the  carbon  box 
are  comparatively  good  conductors  of  heat,  and  consequently 
help  to  distribute  the  heat  evenly  within  the  crucible.  On  no 
account,  however,  should  lamp  black  or  carbon  of  that  nature 
be  used  for  packing,  as  it  conducts  heat  badly,  and  will  take  a 
very  long  time  to  cool  down  afterwards. 

Various  forms  of  pyrometers  have  been  proposed  for  tem- 
peratures above  that  for  which  the  one  described  is  suitable. 
These  usually  depend  on  the  increase  in  the  electrical  resistance 
of  a  platinum  wire  with  the  temperature,  and  of  course  involve 
an  ohmmeter  or  some  other  electrical  measuring  apparatus. 
Such  a  pyrometer  might  be  useful  in  certain  cases,  but  for 
ordinary  work  it  is  not  really  needed.  Good  soft  coal  should 
be  used  in  a  furnace  of  the  kind  described,  as  the  temperature 
is  more  easily  regulated  than  with  hard  coal  requiring  a 
stronger  draught,  and  the  highest  temperature  is  more  easily 
obtained. 

Petroleum  oil  has  been  proposed  as  the  fuel  for  carbonising 
furnaces,  but  the  Author  is  not  aware  that  it  is  used  in  any 
factory.  Petroleum  or  gas  furnaces  might  be  designed  to 
give  very  regular  and  even  heating,  but  would  probably  not  be 
good  for  high  temperatures.  A  very  high  temperature  can,  of 
course,  be  obtained  by  using  ordinary  coal  gas  with  an  air  blast, 
but  it  is  an  expensive  and  troublesome  method,  and  it  is  also 
very  difficult  to  obtain  an  even  heating.  One  part  of  the 
crucible  may  be  nearly  melting  while  another  is  comparatively 
cold. 

As  the  temperature  at  which  a  filament  is  run  after  it  is  in 
the  lamp,  and  also  during  the  process  of  manufacture  subse- 
quent to  carbonisation,  is  greater  than  that  which  can  be 


30  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

obtained  in  the  furnace,  it  might  be  supposed  that  there  would 
be  no  need  to  carbonise  at  a  higher  temperature  than  a  red 
heat,  sufficiently  high  only  to  make  the  filaments  conductors 
of  electricity,  the  carbonisation  being  completed  by  the  subse- 
quent heating  with  the  electric  current.  This,  of  course, 
would  give  a  considerable  saving  in  the  cost  of  the  furnace 
repairs  and  renewals  of  the  crucibles.  Although  such  a 
method  might  perhaps  answer  in  certain  cases,  it  will,  how- 
ever, generally  be  found  that  the  higher  the  temperature 
attained  in  the  furnace  the  better  will  be  the  filaments,  and 
also  the  more  uniform  will  be  the  results.  The  specific 
resistance  of  filaments  baked  at  a  red  heat  is  much  greater 
than  those  which  have  been  raised  to  a  white  heat.  Filaments 
of  different  bakings,  which  have  all  been  brought  to  a  bright 
white  heat  will  be  more  nearly  alike  in  specific  resistance  than 
those  baked  only  at  a  bright  red  heat.  In  the  latter  case,  not 
only  are  the  filaments  of  separate  bakings  liable  to  differ  from 
each  other,  but  the  filaments  from  the  same  crucible  may  be 
found  to  vary  in  specific  resistance.  The  subsequent  heating 
by  the  electric  current  will  cause  the  specific  resistance  to  fall. 
It  will  do  so  even  with  the  filaments  which  have  been  car- 
bonised at  the  highest  possible  temperature  in  the  furnace. 
But  even  then,  after  the  application  of  the  current,  those  car- 
bonised at  the  highest  furnace  temperature  can  be  the  more 
easily  brought  to  uniformity.  Especially  is  this  the  case  if 
the  filaments  are  to  be  flashed.  If  the  filaments  are  uneven 
before  flashing,  they  are  much  more  likely  to  be  uneven  after 
flashing  than  if  they  started  uniform  and  of  a  definite  specific 
resistance,  even  though  that  specific  resistance  be  not  the 
final  one. 

Having  got  the  crucibles  safely  from  the  furnace  to  the  bench 
where  they  are  to  be  unpacked,  the  first  thing  to  be  done  is  to 
remove  the  lids.  This  can  usually  be  accomplished  without 
difficulty,  though  it  is  sometimes  necessary  to  use  a  good  deal 
of  force,  owing  to  the  lid  having  become  fused  in  places  to  the 
crucible.  Care  must  be  taken  that  the  crucible  is  not  knocked 
or  jolted  about.  The  lid  being  off,  it  will  first  be  noticed  that  the 
powdered  carbon  has  sunk  considerably,  the  space  between 
the  carbon  box  and  the  crucible  being  no  longer  full  as  it  was 
when  the  lid  was  put  on.  This  subsidence  is  due  to  the  air 


CARBONISATION.  31 

which  was  originally  imprisoned  by  the  carbon  having  been 
driven  out,  and  not  to  any  loss  of  the  carbon  by  burning 
or  otherwise.  If  through  insufficient  luting  of  the  lid  some  of 
the  carbon  powder  has  been  burnt,  it  will  be  at  once  apparent 
by  a  brown  or  yellow  powder  upon  the  top,  which  is  the  ash 
always  left  by  the  wood  charcoal  such  as  is  used.  No  ash, 
however,  is  usually  seen,  and  no  carbon  dust  has  been  burnt. 

It  must  be  borne  in  mind  that  the  filaments  now  in  the 
crucibles  are  very  different  things  from  what  they  were  when 
put  in.  They  are  now  very  brittle  and  easily  broken.  Con- 
sider for  a  moment  the  condition  in  which  they  now  are,  and 
it  will  be  readily  understood  that  the  greatest  care  must  be 
used  in  handling  them.  The  parchmentised  thread  and  the 
wooden  sticks  between  the  two  parts  of  the  carbon  block  are 
now  carbonised  and  have  shrunk,  so  that  the  top  or  larger 
piece  of  the  block  has  sunk  down  upon  the  lower  part.  The 
parchmentised  thread,  now  carbon,  is  still  in  one  length 
wound  round  and  round  the  carbon  block.  It  is,  therefore, 
evident  that  it  will  not  do  to  lift  out  the  block  in  the  reverse 
manner  to  that  by  which  it  was  put  in.  If  this  is  done,  the 
top  or  larger  block  being  lifted  out,  the  filaments  will  probably 
not  have  the  strength  to  raise  the  lower  one,  which  will 
remain  at  the  bottom  of  the  box,  having  broken  through  all 
the  filaments.  This  would  not  matter  if  they  were  all  to 
break  at  the  lower  end.  It  is,  unfortunately,  an  invariable 
rule  with  filaments  to  break  at  a  point  which  will  render  them 
useless.  The  sliding  carbon  partitions,  however,  provide  an 
easy  method  of  safely  withdrawing  the  blocks.  The  carbon 
box  is  lifted  out  of  the  crucible,  and  is  gently  turned  on  its 
end,  the  lid  having  been  first  removed.  Each  block  of  fila- 
ments now  rests  on  one  of  the  thin  carbon  plates.  These 
are,  then,  carefully  withdrawn  one  by  one  with  the  blocks  of 
filaments  lying  flat  upon  them. 

The  next  thing  to  be  done  is  to  get  the  filaments  off  the 
blocks.  Great  care  must  be  taken  in  handling  the  blocks  at 
this  stage,  because  the  two  parts  are  only  held  together  by 
the  filaments  themselves,  the  wooden  sticks,  which  originally 
fitted  tightly  into  the  carbon  blocks,  having  shrunk  during 
the  carbonisation  so  as  to  become  quite  loose  in  the  holes, 
and  no  longer  serve  to  hold  the  two  parts  together.  If  the 


32  THE    INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 

blocks  were  wrapped  in  paper  or  calico,  the  now  carbonised 
wrapping  must  be  undone.  This  is  easily  accomplished  by 
cutting  it  along  the  edge  of  the  block  with  a  knife,  when 
it  can  be  turned  back  and  the  filaments  exposed  to  view. 
With  a  pair  of  scissors  the  upper  side  of  all  the  filaments 
may  now  be  cut  across  between  the  two  parts  of  the  carbon 
block.  Then  with  a  knife  the  lower  side  of  the  filaments 
may  be  cut  downwards  against  the  carbon  plate  upon  which 
they  are  lying.  The  round  end  piece  of  the  block  can,  there- 
fore, be  removed  with  the  tail  ends  of  the  filaments.  The 
large  part  of  the  block  can  now  be  lifted  up  with  all  the  fila- 
ments hanging  upon  it.  They  should  be  perfectly  straight, 
and  if  they  have  been  skilfully  handled  there  will  often  be  not 
so  much  as  a  single  breakage. 

They  may  now  be  tipped  off  the  block  into  a  suitable  tray 
or  box  and  put  away  until  wanted.  They  must,  however,  be 
kept  in  a  perfectly  dry  place  or  air-tight  box  if  they  are  to 
be  kept  for  any  length  of  time,  as  they  may  absorb  moisture, 
with  the  result  that,  when  they  come  to  be  heated  by  a 
current  of  electricity,  they  may  break,  or  pieces  may  chip  off, 
owing  to  the  sudden  generation  of  minute  quantities  of  steam. 
If  filaments  are  allowed  to  get  into  this  state,  the  moisture 
may  be  removed  safely  by  slowly  baking  them  at  a  tempera- 
ture less  than  red  heat. 

Looped  filaments  are  taken  off  the  blocks  in  a  similar  way. 
The  filaments  are  cut  midway  between  the  loops  and  are  then 
gently  pulled  off  the  carbon  blocks.  There  is  no  difficulty  in 
unpacking  flat  filaments  carbonised  between  paper  or  calico. 
Spiral  and  other  forms  carbonised  loose  in  powdered  charcoal 
are  more  difficult  to  withdraw  without  breakage. 


CHAPTER   IV. 


MOUNTING. 

THE  process  of  joining  the  carbon  filament  to  the  leading-in 
wires  (the  wires  which  support  the  filament  and  convey  the 
current  of  electricity  through  the  glass)  is  usually  called 
"  Mounting."  It  may  be  carried  out  in  a  variety  of  ways, 
some  of  which  will  now  be  described. 

Platinum  wire  is  always  used  to  carry  the  current  of  elec- 
tricity through  the  glass  bulb  to  the  filament.  Platinum  is 
employed  because  it  has  a  coefficient  of  expansion  nearly  the 
same  as  that  of  the  glass  which  is  used,  and  because  it  will 
stand  the  necessary  heating  in  the  blowpipe  flame  without 
melting  or  oxydising  during  the  process  of  "  sealing-in." 
Other  metals  having  a  different  coefficient  of  expansion  will 
not  do,  as  the  glass  will  crack  at  the  seal.  Various  means  have 
been  tried  to  obviate  the  use  of  platinum,  on  account  of  ex- 
pense. Several  experimenters  have  claimed  to  have  produced 
alloys  of  other  metals  which  may  be  used  as  a  substitute  for 
platinum.  Alloys  having  about  the  same  coefficient  of  expan- 
sion as  glass  can  undoubtedly  be  produced,  but  they  are  open 
to  the  objection  that  they  are  unable  to  stand  the  temperature 
of  melting  glass  or  the  action  of  the  blowpipe  flame  without 
melting  or  burning,  so  that,  even  if  the  glass  does  not  crack, 
there  is  likely  to  be  a  leak  from  the  imperfect  fusing  of  the 
glass  to  the  wire.  In  order  to  make  a  safe  seal  it  is,  con- 
sequently, necessary  to  make  a  very  long  one,  and  extra  time 
and  trouble  are  required.  No  such  method  is  likely  to  super- 
sede platinum,  as  the  supply  of  this  metal  appears  to  increase 
with  the  demand.  The  high  price  to  which  it  rose  a  few  years 
ago  was  not  maintained,  and  it  seems  probable  it  will  continue 

D 


34  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

to  fall.  It  takes  only  a  small  amount  of  extra  labour  and 
a  few  failures  to  cost  as  much  as  the  very  small  amount  of 
platinum  which  is  really  necessary.  If  a  cheap  alloy  could  be 
produced,  which  could  be  used  in  exactly  the  same  way  as 
platinum  is  now  used,  it  would,  no  doubt,  very  soon  take  the 
place  of  that  metal  altogether,  but  it  is  a  question  if  any  sub- 
stitute requiring  a  more  elaborate  seal  will  be  able  to  do  so. 
The  platinum  need  only  enter  the  lamp  for  a  short  distance. 
If  it  is  desired  that  the  filament  be  placed  some  distance  away 
from  the  seal,  copper  wires  may  be  fused  to  the  platinum  and 
the  filament  joined  to  the  copper.  Thin  copper  and  platinum 
wires  can  be  easily  fused  together.  The  ends  of  the  wires 
are  held  in  a  small  pointed  blowpipe  flame.  The  copper 
quickly  melts  and  a  small  bead  is  formed.  While  this  bead  is 
liquid  the  hot  end  of  the  platinum  wire  is  stuck  into  it  and 
both  are  removed  from  the  flame  when  it  is  found  that  they 
are  firmly  united.  If  the  wire  within  the  bulb  is  required  to 
be  of  some  length,  it  will  be  necessary  that  it  be  supported, 
or  the  weight  of  the  filament  may  cause  it  to  bend  over.  It 
is  easy  to  stiffen  the  wires  by  the  aid  of  glass.  A  coating  of 
glass  may  be  fused  on  the  wires,  or  a  T  piece  of  glass  may 
be  used,  so  that  the  ends  of  the  cross-piece  support  the  wires, 
the  tail  of  the  T  being  fused  to  the  bottom  of  the  bulb. 

The  joint  between  the  leading -in  wires  and  the  filament 
may  be  a  simple  mechanical  one,  the  filament  being  clamped  in 
some  way  to  the  wires.  A  socket  can  be  formed  on  the  ends 
of  the  wires  and  the  filament  inserted  and  the  socket  squeezed 
tightly  upon  the  filament.  If  the  filament  be  flat  it  may  be 
held  to  the  flattened  ends  of  the  wires  by  a  small  bolt  and  nut. 
A  split  holder,  with  a  ring  on  it  exactly  like  a  small  crayon- 
holder,  such  as  is  used  by  artists,  may  be  used.  In  each  of  these 
methods  the  filament  is  held  simply  by  mechanical  pressure. 

All  these  methods,  which  were  used  in  the  early  days  of 
lamp-making,  have,  however,  been  abandoned  as  unsatisfac- 
tory or  too  costly.  The  filaments  were  apt  to  work  loose, 
owing  to  expansion  and  contraction  with  temperature.  In 
the  earlier  forms  of  the  Lane-Fox  lamp  a  hollow  carbon  tube 
was  used,  into  which  the  platinum  wire  was  inserted  at  one 
end  and  the  filament  at  the  other,  with  some  kind  of  carbon 
paste  to  make  the  joint  firm.  Fig.  10  shows  an  early  form  of 


MOUNTING. 


35 


Swan  lamp  with  the  crayon-holder  joint ;  Fig.  11  shows  a 
Lane-Fox  lamp ;  and  Fig.  12  shows  a  Maxim-Weston  lamp, 
with  platinum  bolt  and  nut. 

Edison  for  a  long  time  made  a  socket  joint,  which  was 
electro-plated  with  copper,  the  deposit  of  copper  extending 


THE  ELECTRICIAN 

FIG.  10. — Early  Swan  Lamp,  showing  Crayon-holder  Joint. 

a  short  distance  along  both  the  wire  and  the  filament.  This 
made  a  good  joint,  both  electrically  and  mechanically,  but  i-fc 
was  abandoned  some  years  ago  in  favour  of  a  simpler  form. 
The  most  common  form  of  joint  is  a  simple  socket  with  a 
deposit  of  carbon  at  the  junction,  in  place  of  the  copper  deposit 
used  by  Edison.  A  simpler  form  is  that  made  by  a  deposit  of 

D2 


36 


THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


carbon  without  any  tube  or  socket.  The  most  simple  form  of 
all,  however,  is  where  the  joint  is  formed  by  a  small  lump  of 
carbon  paste.  This  was  used  by  Edison  after  the  electro- 
plated joint  had  been  given  up.  The  deposited  carbon  joint 
is  the  most  satisfactory  as  a  joint,  and  is  consequently  the 
most  widely  adopted,  though  it  is  more  troublesome  and 


THE  ELECTRICIAN 

FIG.  11. — Eaily  Lane-Fox  Lamp, 
showing  Carbon  Tube  Joint. 


THE  ELECTRICIAN 


FiG§  12. — Early  Maxim-Weston  Lamp, 
'Showing  Bolt  aud  Xut  Joint. 


expensive  to  make  than  the  paste  joint.  The  deposited  joint 
is  made  by  heating  the  junction  of  the  filament  and  wires  by 
electric  current  in  an  atmosphere  of  hydrocarbon  gas,  or 
under  a  hydrocarbon  liquid.  The  gas  is  seldom  used,  as  it  is 
too  slow.  The  liquid  gives  a  much  more  rapid  deposit,  but 
has  the  disadvantage  of  getting  more  or  less  all  over  the 


MOUNTING.  *>  < 

carbon  and  wires,  and  must  consequently  be  driven  off  again 
before  the  filament  is  sealed  into  the  bulb. 

The  first  thing  to  be  done  towards  making  the  joint  is  to 
prepare  the  platinum  wires.  The  wire  has  first  of  all  to  be 
cut  up  into  the  lengths  required.  A  deposited  or  paste  joint 
can  then  be  made  without  any  further  work  on  the  platinum, 
but  it  is  better  to  prepare  the  platinum  in  some  way,  or  the 
cement  will  not  have  a  good  hold  upon  the  wire.  The  plain 
wire  is  too  smooth,  and  should  be  roughened  or  shaped  so 
that  the  carbon  cement  has  something  to  hold  on  by.  The 
wire  may  be  twisted  into  a  spiral,  forming  a  socket  into  which 
the  end  of  the  filament  is  put.  The  spiral  can  be  formed  by 
twisting  the  wire  with  a  machine  like  that  shown  in  Fig.  13. 


FIG.  13.— Wire-Twisting  Machine. 

A  A  is  a  spindle,  with  a  needle  B  at  one  end  and  a  handle  C 
at  the  other.  There  is  a  screw  thread  on  A  which  allows  the 
needle  to  be  rotated  the  same  number  of  times  (two  or  three 
usually)  for  each  wire.  The  end  of  the  platinum  wire  is  in- 
serted in  a  small  hole  close  to  the  needle.  The  wire  is  held 
tightly,  and  the  spindle  rotated  by  the  handle,  thus  twisting 
the  wire  two  or  three  times  round  the  needle.  The  wire  is 
then  slipped  off  the  needle,  straightened,  and  finished  off  with 
a  small  pair  of  pliers. 

If  a  socket  is  required,  it  is  better  to  make  the  tube  form, 
which  uses  less  platinum  than  the  spiral.  To  make  the  tube, 
the  wires  are  first  flattened  out  for  an  eighth  of  an  inch  or  so 
at  the  ends.  This  can  be  done  by  means  of  rollers,  which 
may  be  arranged  to  cut  the  wires  at  the  same  time.  The 
wire,  being  fed  into  the  machine  in  a  continuous  length,  is  cut 


38  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

and  flattened  at  one  operation.  The  wires  are  then  drawn 
through  an  ordinary  wire  draw-plate,  which  bends  the 
flattened  ends  round  into  a  tube.  Before  drawing  through 
the  plate,  the  flattened  ends  must  be  annealed  by  heating  to 
redness  in  a  Bunsen  gas  flame.  If  this  is  not  done  these 
ends  will  be  pulled  off  instead  of  being  formed  into  a  tube. 
The  filament  is  slipped  into  the  socket,  which  is  then  squeezed 
upon  it  so  as  to  hold  it  tightly.  It  is  convenient  to  have  the 
wires  joined  together  by  a  bridge  of  glass  before  mounting 
in  the  socket,  but  it  is  not  necessary. 

If  a  butt  or  lap  joint  is  to  be  made,  the  wires  may  be  nicked 
near  the  end,  so  as  to  form  an  indentation  by  which  the  deposit 
can  hold  on.  An  excellent  way  of  making  a  butt  joint  is  that 
which  is  adopted  by  the  Edison- Swan  Company.  A  small, 
flat  head,  like  a  pin-head,  is  formed  on  the  end  of  the  wires. 
The  end  of  the  filament  is  placed  against  the  centre  of  this 
head,  and  carbon  is  deposited  over  the  head  and  the  end  of 
the  filament.  As  the  carbon  is  deposited  so  as  to  completely 
cover  up  the  head,  it  forms  a  very  secure  joint.  Deposited 
carbon  will  always  hold  tightly  to  the  filament.  For  filaments 
taking  up  to  an  ampere  of  current,  platinum  wire  of  0'014m. 
diameter  is  generally  used.  The  Author  has,  however,  known 
lamps  taking  several  amperes  to  work  perfectly  well  with  this 
size  of  wire.  For  currents  above  one  ampere  larger  wire 
should  be  used,  and  for  more  than  five  amperes  several  small 
wires  are  better  than  one  large  one. 

The  deposited  carbon  joint,  as  has  already  been  mentioned, 
is  made  by  strongly  heating  parts  of  the  wires  and  the  fila- 
ments by  a  current  of  electricity  in  a  hydrocarbon  vapour  or 
liquid.  The  hydrocarbon  is  decomposed  by  the  heated  wire 
and  filament,  and  a  deposit  of  carbon  upon  the  part  heated  is 
the  result.  In  order  to  make  such  a  joint  it  is  necessary  to 
fix  the  wires  and  filament  in  a  machine  which  will  make  the 
necessary  electrical  connections.  For  filaments  which  are 
mounted  in  sockets  the  illustration  Fig.  14  shows  a  convenient 
apparatus.  The  filaments  being  already  clamped  in  the  sockets 
on  the  wires,  the  wires  are  inserted  under  the  springs  A  A, 
which  can  be  raised  by  the  pins  B  B  (one  only  of  these  two 
springs  with  its  pin  can  be  seen  in  the  illustration).  The  metal 
pieces  CC,  which  turn  stiffly  about  DD,  are  turned  so  that 


MOUNTING. 


39 


the  parts  E  E  lightly  press  on  the  filament  at  a  short  distance 
beyond  the  platinum  wire.  The  spring  contact  pieces  F  F  are 
then  lowered  by  the  thumbscrews  G  G,  so  that  they  press 
upon  the  filament  just  over  the  places  where  it  is  supported 
by  EE.  A  good  contact  is  thus  secured  upon  the  filament 
without  straining  it.  The  body  of  the  instrument,  K,  may 
be  made  of  wood,  and  all  the  metal  pieces  may  be  of  brass. 
Between  the  springs  A  A  and  the  wood  K  are  metal  strips  L  L, 
which  are  fastened  to  K  and  are  continued  beyond  K  for  some 


FIG.  14.— Cementing  Machine  for  Socket  Joints. 


distance.  The  current  enters  and  leaves  the  apparatus  by 
these  pieces,  which  may  conveniently  terminate  in  a  hook  H. 
The  path  of  the  current  is  from  the  hook  H  along  L  and  A 
to  the  platinum  wire,  along  the  filament  to  F,  round  F  to  the 
other  side  of  the  filament  and  to  the  other  platinum  wire, 
and  back  through  the  corresponding  parts  on  the  other  side 
of  the  instrument  to  the  second  hook  H.  The  larger  part 
of  the  filament  is  thus  short  circuited  by  FF,  and  the  process 
is  consequently  sometimes  called  "short-circuiting." 

If  a  butt  or  a  lap  joint  is  to  be  made,  the  machine  must 
be  arranged  so  that  the  filament  can  be  easily  and  quickly 


40 


THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


adjusted  to  its  position  opposite  the  platinum  wires.  For  this 
purpose  an  instrument  like  that  shown  in  Fig.  15  will  answer 
the  purpose. 

There  are  two  sets  of  spring  contacts  A  A  and  B  B, 
mounted  on  a  wood  block  C,  of  which  that  part  between  the 
two  sets  of  springs  is  cut  away  to  the  depth  of  about  half  an 
inch.  The  electrical  connections  are  made  to  the  spring  con- 
tacts A  A  by  means  of  the  wire  D  and  a  corresponding  wire 
on  the  other  side  of  the  instrument.  The  platinum  wires  are 
first  clamped  in  A  A,  and  the  filament  is  then  put  into  B*B, 
so  that  its  ends  touch  the  ends  of  the  platinum  wires.  The 
part  which  gets  heated  by  the  current  is,  of  course,  that  be- 
tween the  two  sets  of  clamps.  The  block  is  cut  away  to  allow 
of  a  free  circulation  of  the  hydrocarbon. 


FIG.  15.— Cementing  Machine  for  Butt  Joints. 

When  the  deposit  is  obtained  from  a  hydrocarbon  vapour, 
the  apparatus,  either  that  of  Fig.  14  or  Fig.  15,  with  the 
filament  and  wires  already  placed  in  position,  is  hung  on  a 
support,  which  also  conveys  the  current.  This  support  is  fixed 
on  a  flat  circular  stand,  which  has  a  groove  running  round  it. 
This  groove  contains  mercury,  and  forms  an  air-tight  joint  for 
the  glass  shade,  which  is  placed  over  the  apparatus.  Common 
coal  gas,  enriched  by  passing  over  benzoline,  gasoline,  or, 
ether,  is  led  into  the  apparatus  through  the  stand,  and  an 
exhaust  pipe  is  also  provided.  The  gas  is  turned  on,  and  when 
sufficient  time  has  been  allowed  for  it  to  fill  the  enclosed 
chamber  the  electric  current  is  turned  on.  A  resistance  is,; 
used,  so  that  the  current  can  be  gradually  increased  as  the 
deposit  proceeds.  The  glass  shade  must  be  weighted,  or  other- 


MOUNTING. 


41 


wise  held  down,  or  it  may  be  lifted  up  by  the  gas  inside.  The 
current  is  regulated  so  that  the  pieces  of  the  filament  between 
the  contacts  and  the  platinum  wires  are  brought  to  a  bright 
red  heat,  and  the  deposit  commences.  It  takes,  however,  a 
long  time  in  this  way  to  get  a  sufficient  deposit  upon  the 
platinum,  and  it  is,  therefore,  better  to  use  a  little  carbon  paste 
on  the  junction.  A  suitable  paste  may  be  made  by  mixing 
powdered  carbon  with  a  solution  of  dextrine  or  caromel,  and 
can  be  applied  with  a  small  brush.  If  this  is  done  the  current 
will  be  increased  much  more  quickly,  and  a  sufficiently  strong 
joint  be  obtained. 

The  liquid  process  is,  however,  more  easily  worked,  and 
makes  a  more  satisfactory  joint.  In  this  process  the  frame 
which  holds  the  filament  and  platinum  wires  is  hung  on 
a  support  which  also  conveys  the  current,  so  that  the  part 
which  is  heated,  and  upon  which  the  deposit  is  to  be  formed, 
is  entirely  below  the  surface  of  the  liquid.  Several  fila- 
ments may  be  cemented  at  the  same  time,  the  frames  being 
arranged  in  series,  with  a  switch  connected  to  each,  so  that 
any  one  of  them  may  be  cut  out  if  necessary.  It  is  more  con- 
venient to  use  a  separate  jar  of  liquid  for  each  filament,  though 
one  large  trough  will  answer  the  purpose.  The  jars  are  set  in  a 
metal  or  stoneware  trough  to  catch  any  spillings  of  the  liquid. 
It  is  also  advisable  that  the  whole  of  the  bench  at  which 
the  work  is  done  be  covered  with  zinc  to  prevent  the  wood  from 
being  soaked  with  the  oil.  An  extinguisher,  to  cover  over  the 
whole  of  the  trough  containing  the  pots  of  liquid,  should  be 
provided  in  case  of  accident.  There  is,  however,  no  danger  from 
fire,  unless  very  inflammable  liquids  are  used.  Any  hydro- 
carbon liquid  will  produce  a  deposit  on  a  carbon  filament  heated 
to  redness  below  the  surface  of  the  liquid,  though  some  liquids 
deposit  very  much  more  quickly  than  others.  Vegetable  oils, 
such  as  olive  oil  and  linseed  oil,  produce  a  good  deposit.  They 
are,  however,  too  viscous  for  use  with  very  fine  filaments,  and 
the  smoke  which  is  produced  has  a  bad  smell.  Turpentine 
deposits  very  rapidly,  but  the  deposit  is  too  soft.  Benzoline 
or  benzine  rapidly  deposit,  but  are  too  dangerous  for  ordinary 
use.  Ether  gives  a  rapid  deposit,  but,  of  course,  cannot  be 
used  in  the  open.  Mineral  oils  are  generally  used.  The  best 
kerosene  oil  gives  a  good  and  hard  deposit,  but  is  very  slow. 


42  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

Crude  petroleum,  a  black  liquid  containing  mineral  oils  of 
various  densities  and  flashing  points,  answers  the  purpose  very 
well.  A  satisfactory  liquid  may  be  obtained  by  mixing  one 
which  deposits  rapidly  with  one  which  does  so  slowly,  but  gives 
:a  hard  deposit ;  such  a  mixture,  for  instance,  as  four  parts  of 
best  (high  flashing  point)  kerosine  to  one  part  of  turpentine. 
This  mixture  gives  a  sufficiently  rapid  and  hard  deposit,  and  is 
not  dangerous  to  use.  Even  when  the  liquid  has  become  very 
hot,  a  lighted  match  applied  to  the  surface  will  be  extinguished. 

While  the  deposit  is  going  on  a  stream  of  dark  smoke  rises 
from  the  liquid.  This  smoke  will  burn  as  it  bubbles  up  out  of 
the  liquid  if  it  be  lighted,  but  it  can  easily  be  blown  out  again. 
To  obtain  a  proper  deposit  the  joint  is  maintained  at  a  bright 
red  heat.  There  is  no  danger  of  firing  the  liquid  so  long  as 
the  joint  is  kept  below  the  surface,  and  the  current  turned  off 
before  it  is  lifted  out.  The  liquid  may  be  fired  by  carelessness 
on  the  part  of  the  operator.  If,  for  instance,  a  joint  breaks, 
as  sometimes  happens  through  excess  of  current  when  the 
•depositing  is  going  on,  and  so  breaks  the  circuit,  the  operator 
may  take  out  the  filament  and  put  in  another,  having  forgotten 
to  turn  the  switch  off.  As  soon  as  the  form  with  the  new 
filament  touches  the  contacts  the  joint  will  light  up,  and  if  it 
is  not  below  the  surface  of  the  liquid,  it  will  certainly  set  fire 
to  the  oil,  if  it  be  an  inflammable  one. 

The  strength  of  current  used  in  making  a  joint  is  many 
times  greater  than  the  joint  will  ever  be  called  upon  to  carry 
afterwards.  In  order  to  join  a  filament  six  mils  in  diameter 
to  platinum  wires  of  fourteen  mils  with  tube  sockets,  the 
deposit  to  extend  for  a  tenth  of  an  inch  along  each  leg  of  the 
filament,  about  twenty  volts  will  be  required  to  start  the  depo- 
sition, i.e.,  to  make  the  ends  of  the  filament  bright  red-hot  under 
the  oil.  As  the  deposition  of  carbon  proceeds,  the  ends  of  the 
filament  become  rapidly  duller.  The  thickness  of  the  filament 
is  increased,  and,  therefore,  the  cooling  surface,  while  the 
resistance  is  at  the  same  time  diminished;  and  as  there  is  a 
regulating  resistance  in  the  circuit,  having  a  considerable 
resistance  compared  with  that  of  the  joint,  the  current  rises 
only  a  very  little.  It  is,  therefore,  necessary  to  cut  out  some 
of  the  regulating  resistance  and  increase  the  current,  so  as  to 
keep  the  joint  at  a  bright  red  heat.  In  this  way  the  current 


MOUNTING.  43 

will  be  gradually  raised  to  at  least  twelve  amperes  before  the 
joint  is  large  enough,  with  a  sufficient  amount  of  deposit  on 
the  platinum.  The  volts  will  fall  as  the  current  is  raised. 
With  the  best  kerosene  oil  it  will  take  ten  minutes  to  make 
this  joint.  If  the  operation  be  hurried  by  turning  up  the 
current  too  fast,  the  joint  will  probably  break,  and  both  the 
filament  and  the  wire  will  be  spoiled  by  the  arc  which  will  be 
formed  at  the  break.  With  one-fifth  part  of  turpentine  added 
to  the  kerosene,  the  time  occupied  can  safely  be  reduced  to  one 
minute  and  yet  produce  a  good  hard  joint. 

A  circulation  of  air  should  be  arranged  in  the  room  where 
the  process  is  carried  on,  in  order  to  carry  the  smoke  and 
fumes  away  from  the  operatives.  The  fumes  from  mineral 
oils  do  not  appear  to  be  unwholesome  as  are  those  from 
animal  or  vegetable  oils,  although  the  smell  is  not  altogether 
pleasant. 

The  resistance  used  for  the  purpose  of  varying  the  current 
may  be  composed  of  a  number  of  carbon  plates  lying  one 
against  another,  with  a  screw  arrangement  for  tightening 
them  ;  or  a  water  resistance  may  be  used.  The  least  trouble- 
some way,  however,  is  to  use  an  alternating  current  with 
choking  coils  instead  of  resistance.  An  ammeter  should  be 
used,  so  that  the  operatives  can  watch  the  rate  of  increase  of 
the  current  and  cut  it  off  at  the  right  point.  When  the  liquid 
is  fresh,  the  current  may  be  regulated  according  to  the  ap- 
pearance of  the  red-hot  joint,  but  when  the  liquid  has  been 
used  for  some  time  and  becomes  black,  the  hot  joint  cannot 
be  seen.  Even  then,  however,  the  regulation  can  be  effected 
by  watching  the  volume  of  the  smoke  which  bubbles  up.  An 
ammeter  is,  however,  more  reliable. 

The  deposit  formed  on  the  platinum  is  due  to  the  plati- 
num being  heated  to  the  required  extent  more  by  conduction 
from  the  hot  ends  of  the  filament  than  by  direct  heating  by 
the  current.  The  deposit,  consequently,  does  not  reach  far 
along  the  platinum,  which  is  kept  too  cool  by  the  liquid  to 
decompose  it. 

After  this  process  the  filaments  and  platinum  wires  will  be 
oily  and  must  be  cleaned.  This  is  easily  done,  as  regards  the 
filament,  by  gently  heating  by  means  of  current,  while  the 
platinum  wires  can  be  heated  in  a  Bunsen  or  blow-pipe  flame. 


44  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

If  a  thick,  sticky  oil  has  been  used  for  depositing' from,  it  may 
be  necessary  to  first  wash  the  filaments  in  kerosine. 

In  making  a  paste  joint,  a  drop  of  the  carbon  paste  is 
applied  to  the  ends  of  the  platinum  wires  with  a  small  brush, 
and  the  filament  is  stuck  into  the  drop,  which  quickly  dries 
and  sets.  The  paste  must  then  be  heated  to  drive  off  the 
volatile  substances  with  which  the  carbon  powder  is  mixed, 
and  the  joint  is  made. 


CHAPTER    V. 


FLASHING. 

THE  process  usually  called  "  Flashing"  will  now  be  con- 
sidered. It  is  a  similar  process  to  the  one  just  described  of 
making  joints  by  the  deposition  of  carbon.  Carbon  is  deposited 
upon  the  whole  of  the  filaments  from  either  a  liquid  or  gaseous 
hydrocarbon.  The  name  of  ''flashing"  was  given  to  the 
process  because  the  filaments  used  to  be  flashed,  i.e.,  lighted 
up  and  extinguished  a  number  of  times  at  short  intervals, 
while  in  a  hydrocarbon  vapour.  The  actual  flashing  in  this 
way  is  not  necessary  to  the  process  except  under  certain  con- 
ditions, and  the  term  will  here  be  used  to  mean  the  depositing 
of  carbon  on  a  filament  by  electrically  heating  it  in  a  hydro- 
carbon liquid  or  vapour. 

In  the  early  days  of  lamp  making,  flashing  was  resorted  to 
for  the  purpose  of  making  the  carbon  filaments  light  up  evenly 
along  their  length.  Early  filaments  were  not  even,  and  would 
light  up  with  bright  and  dull  spots  all  over  them.  The  process 
of  flashing  had  the  effect  of  reducing  this  unevenness,  and,  if 
carried  on  for  a  sufficient  length  of  time,  of  entirely  removing 
it.  The  reason  is  simple.  The  bright  spots  on  the  filament  are 
mainly  due  to  those  parts  having  a  higher  electrical  resistance 
than  the  rest,  and  the  dull  spots  to  a  lower  resistance.  This 
variation  in  the  resistance  at  different  parts  of  the  filament 
may  be  due  to  a  variation  in  the  specific  resistance  of  the  fila- 
ment, or  to  a  variation  in  the  thickness,  or  a  combination  of 
both  these  causes. 

Now,  carbon  is  not  deposited  from  a  hydrocarbon  until  a 
certain  temperature  is  reached,  at  which  the  hydrocarbon  is 
decomposed,  and,  up  to  a  certain  point  at  any  rate,  the  higher 


46  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

the  temperature  the  more  rapidly  does  the  deposition  take 
place.  Thus,  when  a  spotty  filament  is  lighted  up  in  the 
presence  of  a  hydrocarbon,  the  higher  resistance  parts,  or 
bright  spots,  will  be  deposited  upon  before  and  more  quickly 
than  the  rest  of  the  filament.  The  result  is  that  the  resistance 
of  those  parts  is  reduced,  and  consequently  they  are  less  and  less 
heated  by  the  current,  and  are  gradually  reduced  to  the  same 
brilliancy  as  the  main  part  of  the  filament.  As  the  current 
is  increased  so  that  the  main  part  of  the  filament  is  hot  enough 
to  be  deposited  upon,  its  resistance  will  fall,  and  it  will  in  turn 
be  gradually  brought  down  to  the  level  of  the  dull  spots. 

Filaments  are,  however,  now  made  by  several  processes 
which  will  light  up  perfectly  evenly  along  their  whole  length, 
and,  therefore,  do  not  require  flashing  for  the  purpose  of 
obliterating  spots. 

Another  difficulty  with  early  lamp-makers  was  to  get  all 
their  filaments  of  the  same  resistance.  The  resistance  of  the 
carbon  filaments  would  vary  very  considerably.  Here,  then, 
another  use  of  the  flashing  process  presented  itself  for  reducing 
different  filaments  all  to  the  same  resistance.  This  can  be 
done  by  connecting  the  filament  during  flashing  with  an  ohm- 
meter,  which  will  show  the  actual  value  of  the  resistance  at 
any  moment.  When  the  resistance  has  fallen  to  the  figure 
required,  the  current  is  cut  off  and  the  process  stopped. 

Another  method  is  to  measure  the  resistance  cold.  A 
three-way  switch  is  used  to  connect  the  filament  alternately 
to  the  dynamo  and  to  a  Wheatstone  bridge,  by  which  the 
resistance  is  measured  cold,  using,  perhaps,  only  one  or  two 
volts.  The  "  bridge"  is  set  for  the  required  resistance,  and 
a  key  is  depressed.  A  reflecting  galvanometer  is  conveniently 
arranged  to  throw  a  bright  spot  of  light  in  front  of  the 
operator,  who  on  depressing  the  key  watches  the  behaviour  of 
the  spot.  If  the  resistance  is  still  too  high  after  the  first 
application  of  the  current,  the  three-way  switch  is  put  over 
again,  and  the  filament  is  again  lighted  up  and  a  further 
deposit  is  given  to  it,  and  so  on  until  its  resistance  is  brought 
down  to  the  required  figure. 

The  actual  flashing  current  may  be  arranged  to  work  a 
Wheatstone  bridge.  The  resistances  are  constructed  to  with- 
stand, without  undue  heating,  the  currents  which  they  will 


FLASHING.  47 

receive,  the  filament  itself  forming  one  arm  of  the  "  bridge." 
A  galvanometer  with  a  conspicuous  pointer  can  then  be  used 
and  fixed  where  easily  seen  by  the  operator,  who  cuts  off 
the  current  at  the  instant  when  the  pointer  indicates  that 
there  is  no  current  passing  in  the  galvanometer  circuit. 

Just  as  filaments  are  now  produced  free  from  spots  without 
being  flashed,  so  are  they  also  produced  all  alike  in  resistance, 
so  that  flashing  is  not  necessary  for  either  of  these  two 
purposes.  Why,  then,  do  lamp-makers  continue  to  use  the 
process?  The  reason  is  that  the  carbon  deposited  by  the 
flashing  process  under  certain  conditions  is  so  much  more 
durable  than  anything  which  can  be  produced  by  other 
methods,  that  it  improves  most  carbons  to  give  them  even  an 
exceedingly  thin  coating  of  deposited  carbon.  It  makes  them 
mechanically  stronger,  and  prevents  the  disintegration  of  the 
filament  from  proceeding  as  rapidly  as  it  otherwise  would  do. 

There  are,  however,  other  effects  of  flashing  which  have  to- 
be  reckoned  with.  One  of  these  is  that  the  filaments  are 
thickened  by  the  deposit  and  their  surface  is  consequently 
increased.  Another  is  that  the  deposited  carbon  may  and 
usually  does  materially  alter  the  emissivity  of  the  surface  ; 
that  is  to  say,  the  rate  at  which  it  will  radiate  light  and  heat. 
Thus  flashing  will  at  the  same  time  reduce  the  resistance 
of  the  filament,  enlarge  the  surface,  and  alter  its  emissivity. 

Now,  to  be  commercially  successful,  a  lamp-maker  must  be 
able  to  turn  out  lamps  which  are  very  nearly  all  alike  in 
candle-power  and  voltage,  when  run  at  the  same  temperature ; 
or,  rather,  when  run  at  the  same  voltage,  the  lamps  must  be 
equally  bright,  small  differences  in  candle-power  not  being 
of  great  consequence.  The  lamp-maker  has  to  aim  at  turn- 
ing out  the  filaments  so  that,  at  the  required  voltage, 
whatever  it  may  be,  they  are  all  equally  bright  and  approxi- 
mately of  the  same  candle-power.  Two  or  three  per  cent, 
variation  in  the  voltage  of  the  lamps  is  the  maximum  which 
should  be  permissable.  A  greater  variation  than  this  will 
be  at  once  apparent  to  the  eye  by  a  difference  in  the  bright- 
ness of  the  lamps.  This  is  a  much  more  important  point 
than  the  actual  candle-power,  i.e.,  quantity  of  light  being 
given  out  by  a  lamp.  A  small  difference  in  colour  or  bright- 
ness can  be  seen  at  once,  whereas  a  difference  of  10  per  cent. 


48  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

in  the  quantity  of  light  given  out  by  two  lamps  running  at 
the  same  temperature  does  not  attract  attention ;  in  fact,  it 
can  only  be  detected  by  the  aid  of  a  photometer. 

Again,  suppose  we  take  two  lamps  running  at  different 
temperatures.  Let  one  be  running  at  two  and  a-half  watts 
per  candle-power,  and  be  giving  a  light  of  ten  candles,  and 
the  other  at  four  watts  per  candle-power,  and  be  giving  fifteen 
candles.  Nine  people  out  of  ten  will  tell  you  that  the  two 
and  a-half  watt  lamp  is  giving  more  light  than  the  fifteen 
candle-power  one,  simply  because  it  is  brighter.  Small 
differences  in  temperature  are  readily  seen,  while  small  varia- 
tions in  the  quantity  of  light  are  not  noticeable.  Hence  a 
lamp-maker  cares  more  about  getting  his  lamps  to  run  at  the 
same  temperature  than  at  the  same  candle-power.  A  uniform 
temperature  for  all  his  lamps  is  the  goal  at  which  a  lamp- 
maker  has  to  aim. 

Now,  as  lamps  are  run  in  parallel,  he  also  has  to  make  his 
lamps  run  at  a  uniform  temperature  at  a  certain  voltage. 
These  are  the  two  fixed  quantities.  The  temperature  may  be 
whatever  he  considers  best  for  the  particular  filament  which 
he  makes,  so  long  as  it  is  the  same  for  all,  and  the  voltage 
must  be  the  voltage  of  the  particular  circuit  for  which  the 
lamps  are  intended. 

The  candle-power  or  quantity  of  light  given  out  may  vary 
to  any  extent  among  different  lamps  in  one  room,  but  if  they 
are  all  at  the  same  temperature  they  will  look  all  right. 

Some  of  the  different  effects  produced  by  flashing  will  now 
be  briefly  considered. 

To  simplify  the  case  as  much  as  possible,  it  will  be  supposed 
that  the  filaments,  after  carbonisation,  are  all  exactly  alike  in 
dimensions  and  in  specific  resistance,  and  in  the  nature  of 
their  surface  or  emissivity. 

Carbon  deposited  on  a  filament  from  a  hydrocarbon  may 
be  of  a  soft,  sooty,  or  of  a  hard  and  dense  kind.  The  latter  is 
the  only  kind  that  is  any  use,  and  will,  therefore,  only  be  con- 
sidered. Such  carbon  has  a  very  much  lower  specific  resist- 
ance than  that  of  the  filaments  themselves.  Consequently, 
with  filaments  of  small  cross  section,  a  very  slight  deposit  will 
reduce  the  total  resistance  very  considerably.  It  is,  therefore, 


FLASHING.  49 

of  the  utmost  importance  that  the  process  be  arrested  instantly 
the  required  resistance  is  reached.  It  is  well  known  that  the 
resistance  of  carbon  falls  as  the  temperature  rises — the  reverse 
of  the  behaviour  of  the  metals.  Roughly  speaking,  the 
resistance  of  an  incandescent  lamp  at  the  temperature  of  the 
air  is  double  the  resistance  it  will  be  when  running  at  the 
ordinary  temperature.  Suppose  that  the  resistance  during 
flashing  is  indicated  by  an  ohmmeter,  and  the  current  be  cut 
off  when  it  has  fallen  to  a  given  number  of  ohms ;  or  let  the 
circuit  be  arranged  Wheatstone-bridge  fashion,  with  a  gal- 
vanometer to  indicate  when  the  required  degree  of  resistance 
has  been  reached.  If  now  a  filament  be  flashed  at  a  moderate 
temperature  until  the  required  resistance  is  indicated,  and  then 
another  filament  be  flashed  at  a  brighter  temperature  to  the 
same  resistance,  and  then  the  two  filaments  be  made  up  into 
lamps  and  tested,  they  will  be  found  to  be  quite  different.  At 
the  same  volts  one  will  be  brighter  than  the  other.  Why  is 
this  ?  Simply  because  the  resistances  were  adjusted  at 
different  temperatures.  If  both  are  run  at  the  same  tempera- 
ture the  resistances  will  be  found  to  differ.  It  is,  therefore, 
obvious  that  this  method  of  flashing  to  a  certain  resistance  is 
useless,  unless  the  flashing  is  done  at  the  same  temperature 
in  all  cases.  The  value  of  the  method,  therefore,  depends  on 
the  skill  of  the  operator  in  adjusting  the  strength  of  current 
by  means  of  a  variable  resistance,  so  that  the  filaments  are  all 
flashed  at  the  same  temperature.  It  is  difficult  to  do  this 
correctly,  as  the  filament  is  at  a  dazzling  white  temperature, 
and  the  operator's  eye  soon  becomes  unreliable.  Of  course,  a 
darkened  glass  or  spectacles  may  be  used,  but  it  is  a  question 
if  the  results  are  any  better.  Another  trouble  is  that  the  glass 
receiver  in  which  the  filament  is  flashed  becomes  coated  with 
a  dark  deposit  which  becomes  thicker  and  obstructs  more 
light  with  each  filament  that  is  flashed,  and  it  must,  conse- 
quently, be  cleaned  very  frequently. 

At  first  sight,  it  might  appear  that  if  the  resistances  of  the 
filaments  be  measured  cold  instead  of  hot,  the  error  due 
to  different  flashing  temperatures  would  be  overcome.  Here, 
however,  another  difficulty  occurs.  Carbon  deposited  at  one 
temperature  is  of  a  different  quality  from  that  deposited  at 
another,  one  difference  being  that  its  resistance  temperature 


50  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

coefficient  is  not  the  same.  Therefore,  two  filaments  flashed  at 
different  temperatures,  so  that  they  measure  the  same  resis- 
tance when  cold,  will  be  found  to  have  different  resistances 
when  hot.  In  this  case  again,  the  results  depend  on  the 
accuracy  of  the  eye  of  the  operator  for  reproducing  the  same 
temperature  during  the  flashing  of  each  filament. 

Apart  from  these  difficulties  in  flashing  to  a  certain  resis- 
tance, either  hot  or  cold,  there  are  many  other  ways  by  which 
the  flashing  may  affect  the  ultimate  evenness  of  the  lamps. 
The  deposit  sensibly  increases  the  thickness  of  the  filaments, 
and  consequently  its  radiating  surface,  and  the  thickened 
filament  will  therefore  require  a  greater  amount  of  electrical 
energy  to  maintain  it  at  the  required  temperature.  If  the 
conditions  of  the  flashing  are  reproduced  exactly  for  each 
filament  the  thickening  will  be  the  same  for  all,  and  can  be 
allowed  for.  If,  however,  a  filament  has  been  flashed  to  a 
certain  resistance  but  has  become  thickened  more  than  the 
amount  allowed  for,  it  will  be  found,  when  made  up  into  a 
lamp,  that  although  its  resistance  may  be  exactly  what  was 
wanted,  yet  it  is  dull  when  run  at  the  voltage  for  which  it 
was  intended.  If  it  be  run  to  the  proper  brilliancy  it  will  be 
found  to  be  of  greater  candle-power  than  was  intended. 

Again,  flashing  almost  invariably  alters  the  emissivity  of  the 
filament,  or  its  power  for  radiating  heat  and  light.  If  the 
conditions  are  precisely  reproduced  in  each  case  a  definite 
emissivity  can  be  calculated  upon.  If  not,  and  there  is  a 
greater  emissivity  than  was  allowed  for,  the  lamps  will  be  dull 
at  the  voltage  for  which  they  were  intended,  and  if  the 
emissivity  be  less  than  that  calculated  upon  they  will  be 
over  bright. 

It  will  now  be  readily  understood  that  extreme  care  is 
needed  in  the  process  of  flashing  if  uniform  results  are  to  be 
obtained.  The  actual  amount  by  which  the  voltage  of  a  lamp 
will  be  affected  by  any  of  these  errors  will  be  dealt  with  further 
on.  In  the  meantime,  some  of  the  processes  of  flashing  will 
be  described. 

The  filaments  may  be  flashed  either  before  they  are  mounted 
or  afterwards,  or  both  before  and  after.  Almost  any  hydro- 
carbon may  be  used,  but  for  each  one  there  will  be  certain 
conditions  which  will  give  the  best  results.  Thus,  one  will 


FLASHING.  51 

work  best  if  the  filament  be  made  very  bright  indeed,  and 
another  may  only  require  a  moderate  temperature.  One  may 
require  to  be  at  a  considerable  pressure,  while  another  must  be 
much  rarefied. 

It  was  stated  in  the  chapter  on  "  Mounting,"  that  a  heavy 
deposit  is  produced  more  quickly  under  a  liquid  than  in  a  gas. 
For  this  reason  it  is  best  to  flash  in  a  gas,  at  any  rate  in  the 
case  of  small-current  filaments,  as  a  thick  deposit  is  not 
required.  As  a  matter  of  fact,  the  deposition  of  carbon  always 
takes  place  from  the  gas,  even  when  done  under  the  surface  of 
a  liquid.  The  liquid  in  contact  with  the  filament  is  intensely 
heated,  and  by  the  time  the  filament  is  red  hot  the  liquid 
round  it  is  boiling  so  violently  that  the  vapour  alone  comes 
into  contact  with  it.  It  is  this  vapour  which  is  decomposed 
and  produces  the  deposit  on  the  filament.  There  is  produced 
in  this  way  a  hydrocarbon  vapour  of  great  density  surround- 
ing the  filament,  and  hence  the  rapid  deposition  of  carbon.  In 
reducing  the  resistance  of  a  filament  by  flashing,  it  is  neces- 
sary that  the  process  be  not  too  rapid,  or  a  slight  error  in 
stopping  the  current  at  the  right  moment  will  considerably 
overstep  the  mark.  For  thin  and  high-resistance  filaments, 
such  as  for  100-volt,  16-c.p.,  or  8-c.p.  lamps,  the  process  must 
be  made  much  slower  than  for  filaments  for  lamps  which  will 
take  several  amperes  of  current.  It  is  consequently  easier  to 
flash  low-current  filaments  successfully  in  a  gas  than  under  a 
liquid,  and  the  gas  may  further  be  considerably  rarefied. 

Ordinary  illuminating  gas  can  be  used  for  flashing  in  at 
atmospheric  pressure.  In  this  case  the  apparatus  required  is 
very  simple,  as  shown  in  Fig.  16.  A  is  a  glass  bell  jar  ground 
flat  on  the  top  and  bottom,  so  that  with  the  aid  of  a  little 
grease  it  stands  gas-tight  on  the  brass  base-plate  B.  Through 
this  plate  pass  the  tubes  C  and  D,  one  being  the  gas  supply 
and  the  other  the  exhaust  pipe.  E  is  a  plate  made  of  hard 
wood,  which  carries  two  spring  clips,  G  G,  for  holding  the 
filament  H.  On  the  upper  side  of  this  plate  are  two  metal 
contact  plates,  FF,  connected  with  G  G,  the  whole  of  this 
arrangement  being  removable.  A  spring  contact  arrange- 
ment, K,  makes  the  electrical  connection  with  F  F,  and  at  the 
same  tune  holds  the  plate  E  firmly  upon  the  top  of  the  bell 
jar  so  that  it  is  gas-tight.  To  work  the  apparatus,  a  filament 

E  2 


52 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


is  put  into  the  clips  which  are  then  inserted  in  the  jar,  and 
the  spring  contacts  are  let  down  on  the  top.  Gas  is  then 
turned  on  through  C,  and  air  and  gas  escape  through  D.  In 
a  short  time  there  will  be  very  little  air  left  in  the  jar,  and 
the  current  may  be  turned  on  and  gradually  increased  by 
means  of  a  regulating  resistance,  until  the  filament  is  brightly 
lighted  up.  The  deposition  of  carbon  now  begins,  and  the 
resistance  of  the  filament  becomes  lower,  and  it  gets  much 


FIG.  16. — Apparatus  for  Coal  Gas  Flashing. 

brighter.  This,  perhaps,  at  first  sight  appears  curious,  as 
it  will  be  remembered  that  when  depositing  on  a  joint  the 
joint  became  dull.  Whether  the  filament  or  joint  becomes 
brighter  or  duller  as  the  deposition  proceeds  simply  depends 
on  the  amount  of  resistance  in  series  with  it.  In  starting  a 
high- resistance  filament  it  probably  requires  all  or  nearly  all 
the  available  electrical  pressure  to  light  it  up,  and  consequently 
the  regulating  resistance  is  mostly  if  not  all  cut  out.  As  the 
resistance  of  the  filament  decreases  the  current  increases,  while 


FLASHING.  53 

the  volts  remain  the  same,  or  nearly  so.  The  filament  conse- 
quently becomes  brighter,  and  the  temperature  must  be  kept 
down  by  adding  resistance.  On  the  other  hand,  if,  when 
the  filament  is  lighted  up,  there  is  a  resistance  in  circuit 
greater  than  the  resistance  of  the  filament,  it  will  become 
duller  as  the  deposition  proceeds,  for,  though  the  current 
through  it  will  increase,  the  difference  of  potential  at  the 
ends  of  the  filament  will  decrease,  so  that  there  is  less  power 
being  expended  in  it.  Some  of  the  regulating  resistance 
must  consequently  be  cut  out  to  keep  the  filament  bright. 
The  total  E.M.F.  used  should  be  just  sufficient  to  light  the 
filament  up  to  the  required  brightness  for  the  deposition, 
without  any  extra  resistance  in  the  circuit.  As  the  deposition 
proceeds  and  the  resistance  of  the  filament  falls,  resistance 
must  be  added  to  the  circuit  in  order  to  keep  the  tempera- 
ture of  the  filament  constant.  More  and  more  resistance 
has  thus  to  be  added  until  the  added  resistance  is  equal  to 
one -fourth  of  the  resistance  of  the  filament  when  first  lighted 
up.  When  this  point  is  reached  the  resistance  of  the  fila- 
ment itself  will  also  be  one  quarter  of  its  original  resistance. 
If  the  deposition  is  continued  beyond  this  point,  the  added 
resistance  is  gradually  cut  out  of  the  circuit  again,  as  the  fila- 
ment henceforth  gets  duller  instead  of  brighter  as  its  resistance 
continues  to  fall.  It  is  here  assumed  that  the  total  E.M.F. 
is  kept  constant,  and  that  the  same  number  of  watts  are  ex- 
pended in  the  filament  throughout  the  process  in  order  to 
maintain  it  at  the  constant  temperature.  As  a  matter  of  fact 
more  power  is  required  as  the  process  proceeds,  owing  to  the 
increase  in  thickness  of  the  filament,  so  that  the  greatest 
amount  of  extra  resistance  required  will  not  be  so  much  as 
one-fourth  of  the  original  resistance  of  the  filament.  It  is 
not  often  that  filaments  are  flashed  to  the  extent  of  reducing 
their  resistance  to  one -fourth  of  the  initial  value. 

The  filament  must,  therefore,  be  kept  at  the  required  bright- 
ness by  the  variable  regulating  resistance  in  series  with  it. 
When  the  ohm-meter  or  galvanometer  indicates  that  the  re- 
quired resistance  is  reached,  the  current  is  immediately  turned 
off,  and  the  filament  is  taken  out  and  a  fresh  one  put  in. 

The  process  of  flashing  in  coal  gas  at  atmospheric  pressure 
is  not  a  good  one.  It  is  too  rapid.  The  deposit  is  very  rough, 


54  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

and  is  apt  to  be  sooty.  A  quantity  of  smoke  is  produced,  and 
the  glass  jar  is  soon  obscured. 

It  is  better  to  reduce  the  pressure  by  means  of  an  exhaust 
pump.  Instead  of  the  exhaust  pipe  D  (Fig.  16)  being  led 
into  the  open  air,  it  is  connected  through  a  stop-cock  to  a 
mechanical  exhaust  pump.  Between  the  stop-cock  and  the 
receiver  is  connected  a  syphon  mercury  vacuum  gauge.  When 
the  filament  has  been  inserted  the  vacuum  cock  is  turned,  and 
a  vacuum  is  produced  in  the  receiver,  which  should  be  ex- 
hausted until  the  vacuum  gauge  shows  a  difference  in  the 
height  of  the  two  sides  of  the  mercury  of  about  lin.  The 
vacuum  cock  is  then  turned  off  and  gas  is  admitted  by  the 
other  cock  until  the  vacuum  is  reduced  to  about  Gin.  dif- 
ference in  level  on  the  gauge.  The  filament  may  then  be 
treated  as  before.  The  deposit  will  be  less  rapid  and  of  a 
better  kind,  being  harder  and  whiter  and  not  so  rough.  If  it 
is  still  too  rapid,  it  can  be  made  slower  by  reducing  the  pres- 
sure again  after  the  gas  has  been  let  in.  This  process  may 
also  be  varied  by  passing  the  gas  over  ether,  gasoline,  benzine, 
&c.,  but  better  results  can  be  obtained  by  methods  which  do 
not  use  illuminating  gas  at  all. 

The  vapour  of  benzine,  ether,  gasoline,  or  other  volatile 
hydrocarbons  may  be  used  alone.  Whichever  is  used,  the 
precise  treatment  will  be  different  in  each  case,  each  hydro- 
carbon requiring  to  be  used  at  a  different  pressure  to  produce 
the  best  results. 

A  method  in  which  the  vapour  of  gasoline  may  be  used  may 
now  be  described.  This  liquid  is  much  used  in  America 
for  the  manufacture  of  gas  for  illuminating  and  other  pur- 
poses in  situations  where  coal  gas  is  not  available.  It  is 
one  of  the  lightest  and  most  volatile  liquids  obtained 
from  petroleum,  having  a  specific  gravity  of  only  about  O66. 
It  has  a  very  disagreeable  smell,  and  is  apt  to  vary  some- 
what in  its  composition,  some  samples  being  more  volatile 
than  others.  For  this  reason  it  is  better  to  use  "  standard 
pentane,"  a  liquid  prepared  specially  for  use  in  the  Harcourt 
standard  lamp.  It  is  simply  gasoline  purified.  It  has  no  bad 
smell,  and  is  always  of  the  same  quality.  It  has  a  specific 
gravity  of  about  0-63.  It  is,  of  course,  a  much  more  expensive 
liquid  than  most  hydrocarbons,  but  it  can  be  used  almost  up 


FLASHING. 


55 


to  the  last  drop  without  waste,  as  it  is  entirely  closed  up  and 
only  admitted  to  the  flashing  chamber  in  exact  quantities 
required  for  use.  As  it  is  extremely  volatile  it  is  necessary  to 
keep  it  in  a  well  stoppered  bottle,  with  the  stopper  fastened 
down,  or  it  will  very  soon  disappear. 

The  process  about  to  be  described,  in  which  pentane  is  used, 
is  capable  of  giving  the  filaments  a  very  thin  coating  of 
deposited  carbon.  A  circular  brass  base-plate,  A  (Fig.  17), 
turned  perfectly  true  and  free  from  flaws,  is  fixed  to  the  bench. 


E 

\i 

E 

F 

D 

„  — 

^ 

v  C 

0 

_  •  , 

>> 

0 

IB  ~TT' 

FIG.  17.  FIG.  18. 

Apparatus  for  Pentane  Flashing. 

A  small  hole,  Jth  of  an  inch  in  diameter,  passes  through  the 
centre.  Into  this  hole  is  screwed  and  soldered  the  end  of  a 
brass  tube  B,  which  passes  through  the  bench  or  table.  Upon 
A  is  a  flat  rubber  ring,  C.  The  glass  receiver  D,  which 
may  be  about  llin.  high  and  2 Jin.  in  diameter,  stands  on 
the  rubber  ring.  The  whole  of  the  clip  arrangement  on  the 
top  of  the  receiver  can  be  lifted  off.  It  consists  of  a  vulcanite 
disc,  E,  underneath  which  is  another  rubber  ring.  Through 
the  disc  pass  two  brass  rods,  G  G,  ending  in  the  spring  ^clips 
H  H,  for  holding  the  filament  F.  Four  of  these  apparatus 


56 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


may  be  fixed  close  to  each  other  on  the  same  bench,  and  can 
be  operated  by  one  person. 

Another  arrangement  is  that  shown  in  Fig.  18.  Two  metal 
rods,  D  D,  pass  through  the  base  plate  A,  from  which  they  are 
insulated,  and  terminate  in  the  hooks  E  E.  A  glass  shade,  F, 
covers  the  whole  and  stands  on  the  rubber  ring  on  A.  The 
filament  is  held  in  a  clip  such  as  that  shown  in  Fig.  19.  The 
metal  parts  C  C  are  insulated  from  each  other  by  mica,  A, 
bound  round  with  metal  B.  The  clip  is  hung  by  the  eye-pieces 
DD  on  the  hooks  EE  (Fig.  18).  Mica  should  be  used  for  the 
insulation,  as  other  insulating  materials  may  not  be  able  to 
stand  the  heating  to  which  they  will  be  subjected  during  a 
prolonged  flashing. 


FIG.  19. — Clip  for  Holding  Filament  during  Flashing. 

Fig.  20  is  a  diagram  of  the  vacuum  connections.  A  A'  B  B' 
are  the  bases  on  which  the  receivers  stand.  The  arrange- 
ment shown  is  such  that  A  A'  can  be  worked  while  B  B'  are 
being  pumped.  There  is  a  syphon  mercury  vacuum  gauge 
CCCC,  about  Gin.  or  Sin.  high,  for  each  receiver.  This 
kind  of  gauge  is  necessary.  A  barometer  tube  gauge  is  too 
large  and  clumsy,  and  is  apt  to  get  broken  when  the  vacuum 
is  destroyed,  owing  to  the  impact  of  the  mercury  with  the  top 
of  the  tube.  Moreover,  the  scale  must  be  a  movable  one,  so 
that  it  can  be  raised  or  lowered  according  as  the  barometer 
rises  or  falls,  or  the  true  state  of  the  vacuum  in  the  receivers 
cannot  be  known.  A  syphon  gauge  always  shows  the  actual 
state  of  the  vacuum,  whatever  the  atmospheric  pressure 


FLASHING. 


57 


happens  to  be.  D  D'  are  two-way  cocks  connecting  A  to  A' 
and  B  to  B'.  E  E'  are  three-way  cocks  which  will  connect 
A  A'  or  B  B'  either  to  the  air  by  F  F,  or  by  the  three-way 
cock  G  to  the  pentane  resevoir  H,  or  to  the  vacuum  pump  K. 
The  method  of  working  is  as  follows  : — 

Filaments  to  be  flashed  having  been  put  into  the  receivers 
on  A  A',  the  cock  D  being  open ;  the  cock  E,  as  shown  in 
the  diagram,  connects  A  A'  to  G,  and  through  G  to  the  vacuum 


C-" 


Section  of 
Three-way  Cock. 


FIG.  20. — Vacuum  Connections  for  Pentane  Flashing. 

pump,  which  may  be  at  a  distance  in  the  engine  room  of  the 
factory,  and  be  driven  by  power.  A  vacuum  is  soon  produced 
in  the  receivers  on  A  A',  and  when  it  arrives  at  about  Gin.  pres- 
sure of  mercury  its  progress  can  be  watched  on  the  gauge  C  C. 
It  is  essential  in  this  method  of  flashing  that  all  joints,  pipes, 
and  cocks  be  absolutely  airtight.  It  might  be  supposed  that 
arrangements  such  as  shown  in  Figs.  17  and  18  would  not  be 
airtight.  Standing  with  their  own  weight  on  the  base-plates 
the  glass  cylinders  would  not  be  tight,  but,  as  soon  as  the 
vacuum  is  turned  on,  the  pressure  of  the  air  forces  the  cylinder 


58  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

so  tightly  against  the  rubber  rings  that  the  arrangement  does 
not  leak,  and,  if  properly  made,  it  will  hold  a  vacuum  for 
hours.  Of  course,  the  arrangement  shown  in  Fig.  18  with 
only  one  rubber  joint  is  less  likely  to  leak  than  the  one 
(Fig.  17)  where  the  filament  is  inserted  through  the  top  of  the 
cylinder. 

The  vacuum  is  applied  until  the  difference  in  level  of  the 
mercury  in  the  gauges  is  about  half  an  inch.  This  is  as  far  as 
most  mechanical  air-pumps  will  go  in  practice.  The  cock  G 
is  then  turned,  disconnecting  the  receivers  A  A'  from  the 
vacuum  pump,  and  connecting  them  with  the  reservoir  H, 
which  has  previously  been  partly  filled  with  pentane.  Pentane 
vapour  now  passes  from  H  along  the  tubes  into  the  receivers 
A  A'.  This  is  apparent  by  the  increased  pressure  indicated 
by  the  gauges  C  C.  The  vapour  should  be  allowed  to  enter 
until  the  gauges  show  Sin.  difference  in  level.  The  cock  G 
is  then  turned  back  again,  connecting  A  A'  with  the  vacuum 
pump,  and  the  vacuum  is  again  reduced  to  about  IJin.  by  the 
gauges.  The  cock  E  is  then  turned  so  that  A  A'  are  discon- 
nected entirely.  The  cock  E'  is  now  turned  so  that  BB'  are 
connected  through  G  to  the  vacuum  pump.  The  condition  of 
A  and  A'  is  now  the  same,  and  the  cock  D  is  turned,  so  that 
A  is  shut  off  from  A'.  Current  is  now  applied  to  the  filament 
in  A.  The  increase  in  temperature  of  the  gas  in  the  receiver, 
produced  by  the  heating  of  the  filament,  causes  increased 
pressure,  and  the  gauge  shows  a  decreasing  vacuum.  The  fila- 
ment is  brought  to  a  bright  white  heat,  at  which  it  is  main- 
tained by  means  of  the  regulating  resistance,  until  the  indicator 
which  is  used  shows  that  it  has  received  a  sufficient  coating  of 
carbon.  The  filament  in  A'  is  then  treated  in  the  same  way. 
The  cock  D  may  then  be  opened,  and  E  turned  so  as  to  admit 
air  through  F  into  A  and  A'.  The  filaments  are  then  taken 
out  and  fresh  ones  put  in.  The  same  process  is  then  repeated 
with  the  filaments  in  B  and  B'.  While  the  flashing  is  going 
on  in  one  side  of  the  apparatus  the  other  side  is  being 
pumped. 

This  may  appear  a  complicated  arrangement  of  pipes  and 
stopcocks,  but  with  a  little  practice  it  can  be  worked  very 
quickly.  With  the  apparatus  shown  in  Fig.  18,  the  electri- 
cal connections  with  the  different  filaments  is  made  by 


FLASHING.  59 

means  of  a  switch,  while  with  the  other  form  (Fig.  17)  one 
clip  on  the  end  of  a  flexible  wire  makes  connection  to  the 
wires  G  G.  By  this  latter  method  the  chances  of  turning  the 
current  on  to  the  wrong  filament  are  less  than  with  the  switch 
arrangement,  as  the  operator  is  apt  to  forget  to  put  the  switch 
over,  whereas  in  the  other  arrangement  the  clip  must  be 
removed  from  the  top  of  the  apparatus  before  the  filament  can 
be  taken  out. 

A  quicker  method  of  flashing  is  to  use  a  large  receiver  in 
which  six  or  eight  or  more  filaments  can  be  flashed  one  after 
the  other  in  the  same  vapour.  This,  however,  is  not  a  good 
plan,  as  the  conditions  are  then  different  for  each  one.  The 
only  way  to  treat  the  filaments  exactly  alike  is  to  do  them 
separately.  For  this  reason  the  cocks  I)  D'  (Fig.  20)  are  put 
between  the  receivers,  so  as  to  prevent  the  gas  and  smoke 
from  one  entering  the  other  as  would  otherwise  happen. 

Immediately  below  A  A'  B  B'  is  a  bulb  filled  loosely  with 
cotton  wool  to  act  as  a  filter,  so  that  pieces  of  filament  and 
dirt  may  not  accumulate  and  block  the  pipes.  There  should 
also  be  another  filter  between  G  and  the  vacuum  pump.  The 
tubing  and  cocks  may  be  either  of  metal  or  glass.  The  latter 
are  more  easily  obtained  and  maintained  vacuum-tight ;  but 
they  are,  on  the  other  hand,  more  liable  to  breakage.  Con- 
nection between  the  cocks  and  tubes  can  be  made  by  rubber 
"vacuum -tubing,"  the  glass  or  metal  tubes  being  first  greased 
with  vaseline. 

The  brown  coating  which  makes  its  appearance  on  the  glass 
receivers  during  flashing  can  be  easily  cleaned  off  with  a  rag 
wetted  with  methylated  spirit. 

The  electrical  connections  will  be  in  accordance  with  the 
method  of  indication  adopted,  either  for  flashing  to  resistance 
measured  hot  or  cold,  or  any  other  system.  Care  should  be 
taken  in  arranging  the  apparatus  that  the  operator  shall  not 
be  liable  to  receive  shocks  A  mercury  double-pole  switch 
which  is  normally  held  off  by  a  spring  is  recommended,  so 
that  the  apparatus  is  only  connected  to  the  dynamo  while  the 
switch  is  held  on.  The  current  is  left  on  continuously  while 
the  depositing  takes  place. 

The  deposit  of  carbon  produced  by  this  method  is  hard 
and  white,  and  of  low  specific  resistance.  By  using  a  greater 


60  THE    INCANDESCENT    LAMP    AND    ITS   MANUFACTURE. 

pressure  and  quantity  of  vapour,  pentane  will  give  carbon 
varying  from  the  hard  white  variety  to  a  soft  black  one. 

The  Author  is  not  aware  that  any  systematic  experiments 
have  been  carried  out  for  the  purpose  of  discovering  what  is 
the  best  arnount  of  deposit  to  give  the  filaments.  Any  thick- 
ness can,  of  course,  be  given  by  prolonging  the  process. 

One  use  of  the  process  of  flashing,  not  yet  mentioned,  is 
that  one  size  (diameter)  of  filament  may  be  made  to  do  duty 
for  lamps  which  are  to  take  currents  varying  considerably  in 
amount  from  each  other.  Thus,  a  filament  suitable  for  a  100- 
volt  lamp  may  by  flashing  be  made  into  a  50-volt  of  the  same 
candle-power,  the  one  size  of  filament  being  available  for  all 
lamps  with  currents  falling  within  such  range.  It  seems 
probable,  however,  that  one  particular  ratio  of  thickness  of 
deposit  to  thickness  of  filament  will  give  the  best  results  for 
all  sizes  of  the  same  filament  flashed  by  the  same  method. 
Experiments  of  this  sort  are,  however,  very  tedious  to  carry 
out,  as  conclusions  can  only  be  arrived  at  by  life  tests  of  a 
number  of  lamps  of  each  of  the  variations. 

The  practical  limit  of  the  use  of  one  size  of  filament  for 
various  currents  is,  however,  determined  by  the  time  taken  to 
flash.  The  longer  the  time  the  more  costly  will  be  the  pro- 
cess, as  fewer  filaments  can  treated  be  by  each  apparatus  and 
operator.  The  cost  of  a  long  flashing  will  soon  mount  up, 
and  be  greater  than  the  extra  trouble  of  making  different  sizes 
of  filaments  at  first.  Double  the  time  of  flashing  means  half 
the  number  of  filaments,  or  twice  the  apparatus  and  number 
of  operators. 

Provided  that  the  receiver  of  the  flashing  apparatus  is  large 
enough  to  hold  plenty  of  hydrocarbon  vapour,  the  thickness 
of  deposit  is  proportional  to  the  time  during  which  the  fila- 
ment is  lighted  up.  This  is  not  correct  for  a  very  prolonged 
flashing,  as  the  hydrocarbon  vapour  becomes  impoverished 
and  works  slower  and  slower,  but  with  a  properly  proportioned 
receiver  it  may  be  taken  as  being  so  within  the  time  which 
can  be  allowed  for  the  flashing.  Time  really  fixes  the  limit  to 
the  amount  of  flashing.  The  pentane  method  described  will 
produce  a  good  and  even  deposit  in  from  twenty  seconds  to 
one  minute.  The  conditions  can  be  arranged  so  that  the 
deposit  proceeds  at  the  rate  of  about  0-OCOOlin.  per  second. 


FLASHING.  61 

In  thirty  seconds  there  will  be  then  a  good  deposit  OOOOSin. 
thick.  A  coating  of  this  thickness  will  be  just  as  smooth  as 
the  filament  upon  which  it  is  deposited.  Prolonged  flashing 
gives  a  rougher  deposit  which  may  have  a  varying  emissivity, 
and,  therefore,  a  thin  deposit  is  better.  It  is  most  convenient 
in  practice  to  give  the  same  thickness  of  deposit  for  all  sizes  of 
filament.  The  same  time  will  then  always  be  taken  in  flash- 
ing, whatever  the  size  of  filament.  The  size  of  filament  must, 
therefore,  be  varied  according  to  the  current  it  is  to  take.  No 
one  size  should  be  used  for  filaments  taking  different  strengths 
of  current,  except  within  very  narrow  limits. 

The  E.M.F.  required  to  flash  filaments  depends  upon  their 
length,  thickness  and  specific  resistance.  For  a  100-volt 
16-c.p.  filament  from  200  to  300  volts  may  be  required  at 
starting,  and  as  much  as  1-6  ampere  may  be  sent  through 
the  filament  during  the  flashing  when  it  will  only  take 
0-64  ampere  when  finished.  This  great  difference  is  due 
chiefly  to  the  cooling  effect  of  the  convection  currents  within 
the  receiver,  which  are,  of  course,  absent  in  the  finished  lamp. 
When  flashing  at  atmospheric  pressure  or  under  a  liquid 
the  difference  is  much  greater.  If  the  filaments  are  not 
properly  carbonised  before  flashing,  a  much  greater  electrical 
pressure  may  be  required  to  light  them  up  in  the  first  instance 
than  that  given  above. 

As  already  mentioned,  the  method  of  flashing  filaments  to 
a  certain  resistance,  even  when  it  is  accurately  done  to  the  hot 
resistance,  does  not  produce  uniform  results  in  voltage  unless 
the  size  and  emissivity  of  the  filaments  are  also  uniform. 
There  is,  however,  another  method  in  which  the  size  and  emis- 
sivity do  not  matter  within  certain  limits.  The  filaments  are 
flashed  directly  to  voltage,  and  the  candle-power  and  resistance 
may  take  care  of  themselves.  Unfortunately,  however,  the  eye  of 
the  operator  has  to  be  depended  upon  for  producing  and  main- 
taining the  right  temperature  even  more  accurately  than  when 
flashing  to  resistance.  The  quantity  of  hydrocarbon  vapour, 
and  particularly  the  pressure,  must  be  exactly  the  same  for  each 
filament.  A  voltmeter  is  connected  so  as  to  indicate  the  volts 
at  which  the  filament  is  being  run.  The  current  is  turned 
on,  and  the  filament  is  brought  as  quickly  as  possible  up  to 
the  required  brightness.  Suppose  that  the  filaments  are  for 


62  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

100  volts  and  16-c.p. :  the  voltmeter  indicates,  say,  200  volts. 
As  the  deposition  proceeds  the  filament  falls  in  resistance, 
but  by  means  of  the  regulating  resistance  it  is  kept  at  the 
same  brightness.  The  indication  of  the  voltmeter  is  all  the 
while  gradually  falling,  and  when  it  reaches,  say,  170  volts 
the  current  is  cut  off.  Now,  provided  that  the  conditions 
of  temperature  or  brightness  and  the  amount  and  pres- 
sure of  the  hydrocarbon  vapour  are  the  same  in  each  case, 
it  does  not  matter  whether  the  filaments  are  15,  16,  or 
17-c.p.  ones  ;  if  the  process  is  stopped  when  the  volt- 
meter indicates  170  volts  they  will  be  alike  in  voltage 
when  made  into  lamps,  170  volts  in  the  flashing  receiver 
being  the  corresponding  value  for  100  volts  in  the  properly 
exhausted  lamp.  The  actual  voltage  at  which  the  current  is 
to  be  cut  off  must,  of  course,  be  found  by  trial  in  the  first 
instance  for  the  particular  apparatus  and  conditions  employed. 
The  degree  of  exhaustion  in  the  receiver  must  be  accurately 
reproduced  each  time.  The  greater  the  pressure  the  greater 
will  be  the  E.M.F.  required.  A  similar  method  can  be  used 
for  making  lamps  to  work  at  the  same  current,  an  ampere- 
meter being  used  instead  of  a  voltmeter. 

Hard,  white,  deposited  carbon  has  only  about  one-tenth  of 
the  specific  resistance  of  carbon  made  from  amyloid,  the 
resistance  of  the  deposited  carbon  of  that  kind  being  about  350 
microhms  per  cubic  centimetre,  and  "that  of  amyloid  carbon 
3,500  microhms  at  a  temperature  of  from  6  to  2  watts  per 
candle  power. 

At  the  ordinary  temperature  of  the  air  the  deposited  carbon 
is  about  two  and  a-half  times  greater,  and  the  amyloid  variety 
one  and  a-half  times  only. 

It  will  thus  be  understood  that  the  ratio  of  the  cold  resist- 
ance to  the  hot  resistance  of  a  flashed  filament  depends 
entirely  on  the  ratio  of  the  amounts  of  the  two  kinds  of 
carbon  of  which  it  is  composed,  a  much-flashed  filament 
having  a  larger  ratio  than  one  only  slightly  flashed.  The 
resistance  of  a  filament  measured  cold  gives  no  indication 
of  its  hot  resistance  unless  that  ratio  is  known.  The  most 
usual  ratio  is  about  two  to  one,  though  it  is  often  above 
or  below  that  amount.  The  specific  resistance  of  arc-lamp 
carbons  (cold)  will  vary  from  13,000  microhms  per  cubic 


FLASHING.  63 

centimetre  to  less  than  that  of  amyloid,  according  to  the 
method  of  manufacture,  moulded  carbons  having  a  higher 
specific  resistance  than  squirted  ones. 

Different  kinds  of  carbon  have  different  emissivities — that 
is,  they  radiate  heat  and  light  at  different  rates,  so  that,  at  a 
given  temperature,  carbons  of  the  same  size  but  of  different 
makes  require  different  amounts  of  power  expended  in  them 
in  order  to  maintain  that  temperature.  This  curious  property 
appears  to  be  connected  with  the  nature  of  the  surface  only, 
and  has  nothing  to  do  with  the  material  underneath  the 
surface. 

It  is  well  known  that  a  hot  polished  copper  rod,  if  covered 
with  lamp  black  will  cool  much  more  rapidly  than  a  similar 
rod  not  blackened.  In  the  same  way  a  hot  lamp  filament 
with  one  kind  of  surface  will  cool  faster  than  a  similar  fila- 
ment with  another  kind  of  surface.  A  filament  made  of  amy- 
loid carbon  will  cool  faster  than  the  same  filament  if  it  be 
flashed  in  pentane  in  the  way  described.  Suppose  that  an 
amyloid  carbon  filament  be  made  into  a  lamp  and  tested,  and 
that  it  is  found  that  at  4  watts  per  candle  power  it  takes  84 
watts  and  gives,  consequently,  a  light  of  21  candles.  Let  the 
same  filament  be  taken  out  of  the  lamp  and  flashed  a  very 
little  by  the  pentane  process,  and  then  be  made  into  a  lamp 
again  and  tested.  Let  it  be  run  at  the  same  temperature  as 
before,  and  it  will  be  found  to  give  only  15  candles  and  take 
only  60  watts.  Now  that  it  is  flashed  and  has  a  different 
kind  of  surface  it  loses  heat  much  more  slowly,  and  a  less  ex- 
penditure of  power  suffices  to  maintain  it  at  the  former  tem- 
perature. It  must  be  particularly  noticed,  however,  that 
there  is  a  corresponding  decrease  in  candle  power.  Let  it  be 
run  at  its  original  candle-power  (i.e.,  21)  and  it  will  be  found 
to  take  about  68  watts,  equal,  that  is,  to  about  3J  watts  per 
candle,  considerably  less  power  than  before  flashing.  Many 
people  have  supposed  that  results  like  this  indicate  that 
the  flashed  filament  is  a  more  efficient  one  than  the  unflashed. 
This,  however,  is  not  at  all  the  case.  The  filament  now  giving 
21  candles  is  at  a  higher  temperature  than  it  was  when  giving 
that  candle  power  before  being  flashed.  If  it  had  not  been 
flashed  at  all,  it  could  have  been  equally  well  run  at  this  tem- 
perature and  efficiency,  only  it  would  then  have  given  about 


64  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

28  candles  and  have  taken  about  90  watts.  The  filament 
has  simply  become  less  emissive :  it  radiates  more  slowly. 

The  extent  of  surface  of  filaments  will  vary  with  different 
kinds  of  surfaces  from  6,000  to  10,000  square  mils  per  candle- 
power  at  the  temperature  of  4  watts  per  candle-power  (equal 
to  100  to  170  c.p.  per  sq.  in.).  When  run  up  to  the  tempera- 
ture at  which  the  best  carbon  begins  rapidly  to  disintegrate 
(0-8  watt  per  candle-power)  as  much  as  1,900  c.p.  per  sq.  in. 
may  be  reached.  The  crater  of  an  arc-lamp  carbon  is  said  to 
give  about  45,000  c.p.  per  sq.  in. 

The  change  in  emissivity  produced  by  flashing  gives  an 
additional  reason  for  carefully  reproducing  the  conditions  under 
which  that  process  is  carried  out  for  each  filament  so  that 
they  may  all  come  out  alike. 

The  actual  amount  of  the  error  in  the  voltage  of  the  lamps 
produced  by  the  several  causes  mentioned  will  now  be  con- 
sidered. 

An  error  in  the  thickness  of  a  filament  is  a  serious  one. 
Suppose  that  a  filament  for  a  100-volt  15-c.p.  lamp  at  4  watts 
per  candle-power  (=  60  watts  total)  should  be  6  mils  in 
diameter,  but  that  the  conditions  of  flashing  have  not  been 
quite  right,  so  that  the  diameter  comes  out  at  6*4  mils,  an 
increase  of  6-6  per  cent,  in  diameter,  and  therefore  in  surface. 
The  filament  will  now  require  6*6  per  cent,  more  energy,  and 
will  be  increased  a  like  amount  in  candle-power.  It  will  take 
64  watts  and  give  16  c.p.  But  it  was  flashed  to  the  resistance 
for  a  15-c.p.  lamp  (166  ohms),  not  for  a  16-c.p.  one.  Conse- 
quently, it  will  take  103*2  volts  and  0-62  ampere,  to  give  the 
right  temperature.  Or  if  run  at  100  volts  it  will  give  only 
about  13*5  candles,  and  an  efficiency  of  4J  watts  per  candle- 
power. 

It  will  be  noticed  that,  though  the  increase  in  diameter  is 
6*6  per  cent.,  the  increase  in  voltage  is  only  about  half  that 
amount,  the  error  being  divided  between  the  E.M.F.  and  the 
current.  The  percentage  error  in  voltage  will  be  about  one- 
half  that  of  the  diameter  for  ordinary  variations. 

With  regard  to  errors  in  emissivity  due  to  the  conditions  of 
flashing  being  wrong,  the  effect  is  exactly  the  same  as  for 
errors  in  thickness,  an  increased  emissivity  having  the  same 
effect  as  an  increase  in  diameter.  A  similar  result  will  be 


FLASHING. 


65 


produced  if  filaments  are  mounted  with  wrong  lengths.  This 
is  usually  looked  upon  as  a  less  serious  matter  than  differences 
in  thickness,  as  it  is  supposed  to  be  easy  to  mount  filaments 


60  80          10Q          120          14O         160          180         200 

Time  of  Flashing ;   Seconds. 

FIG.  21. — Curves  showing  Effect  of  Flashing  on  the  Resistances  of 
Filaments  of  Various  Diameters. 

to  the  proper  length.  Suppose  a  filament  is  to  be  5in.  long. 
One  per  cent,  is  O05in. — that  is  to  say,  it  must  be  mounted 
with  an  error  of  not  more  than  O025in.  on  each  leg,  in  order 


20 


40 


60  80  100          120 

Time  of  Flashing;    Seconds. 


200 


FIG.  22. — Curves  showing  Effect  of  Reduction  in  Resistance,  due  to 
Flashing,  on  the  Voltage  of  Filaments  of  Various  Diameters. 

to  be  within  1  per  cent.     It  is,  therefore,  very  important  to  be 
exact  in  the  matter  of  length. 

With  errors  of  resistance  the  same  kind  of  result  occurs  as 
with  errors  in  surface.      One  per  cent,  error  in  resistance  will 

F 


66 


THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


produce  only  about  one-half  per  cent,  error  in  voltage.  This 
must  not  be  understood  to  apply  to  very  large  differences  in 
resistance.  For  instance,  a  50  per  cent,  reduction  in  resistance 
gives  more  than  25  per  cent,  reduction  in  volts.  The  actual 


360 


40  60  80  100          120          140 

Time  of  Flashing,    Seconds. 

FIG.  23. — Curves  showing  Effect  of  Reduction  in  Resistance,  due  to 
Flashing,  on  the  Current  for  Filaments  of  Various  Diameters. 

effect  on  the  volts  and  current  of  a  gradually  diminishing 
resistance  is  shown  in  the  accompanying  curves,  Figs.  21, 
22,  23.  The  resistance  is  supposed  to  be  diminished  by 
flashing,  the  thickness  of  the  deposit  of  flashed  carbon  being 


FLASHING.  67 

•considered  as  proportional  to  the  time  of  flashing,  the  rate 
of  flashing  being  such  that  a  4-mil  filament  is  reduced  50  per 
cent,  in  resistance  in  ten  seconds. 

It  must  be  understood  that  the  effect  of  reduction  in  resis- 
tance alone  is  shown  in  these  curves.  The  actual  results  of 
flashing,  taking  into  consideration  the  change  in  resistance, 
emissivity  and  thickness,  are  fully  dealt  with  in  a  subsequent 
chapter. 

It  has  been  shown  that  the  time  of  flashingjis  an  important 
consideration,  and  that  it  is  better  to  flash  always  to  about  the 
same  length  of  time,  and  to  use  filaments  of  different  diameters 
which  will  permit  of  this  being  done,  than  to  use  only  a  few 
sizes  of  filaments  and  to  vary  the  time  of  flashing. 

It  is  therefore  necessary  to  consider  the  relation  of  the 
sizes  of  filaments  to  the  candle-power,  voltage  and  tempera- 
ture at  which  they  are  to  be  run. 


F  2 


CHAPTER  VI. 


SIZES  OF  FILAMENTS  (UNFLASHED). 

A  HOT  BODY  may  lose  heat  in  three  ways  :  (1)  by  conduction 
to  another  body  with  which  it  is  in  contact ;  (2)  by  convec- 
tion currents  in  the  medium  by  which  it  is  surrounded,  or 
(3)  by  radiation. 

The  filament  of  an  incandescent  lamp  practically  loses  heat 
by  radiation  alone.  Some  is  lost,  no  doubt,  by  conduction 
along  the  supporting  wires,  whence  most  of  it  is  radiated 
at  a  lower  temperature,  a  little  being  conducted  away  to  the 
fastening  outside  the  lamp.  The  proportion  lost  by  conduc- 
tion in  this  way  is  insignificant,  except,  perhaps,  in  the  case 
of  large  current  small  candle-power  lamps,  and  need  not, 
therefore,  be  considered.  In  a  properly  exhausted  lamp  the 
loss  by  convection  will  be  practically  nil.  Radiation,  there- 
fore, may  be  considered  to  account  for  the  whole  of  the  loss 
of  heat. 

The  rate  of  loss  of  heat  of  a  conductor  by  radiation  is  pro- 
portional to  its  surface.  When  a  constant  temperature  is 
maintained  in  the  conductor,  the  rate  of  loss  of  heat  must  be 
equal  to  the  rate  at  which  heat  is  generated  in  the  conductor. 
If  heat  is  generated  at  a  greater  rate  the  temperature  will  rise, 
if  at  a  less  rate  it  will  fall. 

If  the  same  temperature  is  to  be  maintained  in  different 
conductors,  the  extent  of  surface  of  those  conductors  must  be 
proportional  to  the  rate  of  generation  of  heat  within  them. 
The  rate  of  generation  of  heat  in  lamp  filaments  is  measured 
in  watts.  Consequently,  in  order  to  maintain  the  same 
temperature  in  different  sized  filaments,  the  extent  of  surface 
of  the  filaments  must  be  proportional  to  the  watts  expended 


70  THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

in  them ;  or,  vice  versa,  the  watts  expended  must  be  propor- 
tional to  the  extent  of  the  surfaces  of  the  filaments.  The 
filaments  must,  offcourse,  be  in  a  good  vacuum,  and  be  of  like 
emissivity. 

The  relation  of  the,'  power  expended  to  the  extent  of  radiat- 
ing surface  is  theVrulingTfactor  upon  which  everything  else 
depends  in  determining  the  size  of  the  filaments.  With  fila- 
ments of  the  same  emissivity  this  relation  will  be  constant  for 
any  given  temperature,  no  matter  whether  the  filaments  be 
long  and  thin  or  short  and  thick,  or  whether  they  are  circular 
or  flat,  solid  or  hollow. 

As  a  rule,  filaments  are  circular  in  section  and  solid,  not 
tubular,  and  this  kind  will,  therefore,  be  first  considered. 

Circular  Filaments. 

Let  d  =  the  diameter  of  the  filament, 

I   =     „    length         „  „ 

r  =     ,,    resistance    ,,  ,, 

s  =     ,,    surface         ,,  ,, 

c  =     ,,    current        ,,  ,, 

and  c2r   =     ,,    power  spent  in  the  filament. 
Now,  as  the  power  spent'is  proportional  to  the  surface,  we  have 

c2  r  oc   s  ; 

the  resistance  r  is  proportional  to  the  length  I  and  inversely 
proportional  to  the  square  of  the  diameter,  or 

I 

rcc   V 

(unflashed  filaments  are,  of  course,  being  considered),  and  the 
surface  s  is  proportional  to  the  diameter,  multiplied  by  the 
length,  or 

s  oc   dl. 

Therefore,  c2  x  -  <x.  dl, 

ct 

which  simplifies  into  c2  oc  dB, 

or  coc  d%, 

and  inversely  d  oc  c§. 

It  is  thus  apparent  that  the  diameter  of  the  filament  must 
be  proportional  to  the  f  power  of  the  current,  or  that  the 


SIZES    OF    FILAMENTS    (UNFLASHED).  71 

current  must  be  proportional  to  the  |  power  of  the  diameter. 
The  length  I  disappears  from  the  equation,  and  we  see  that  for 
any  given  temperature  of  the  filament  the  diameter  is  deter- 
mined solely  and  only  by  the  current  which  it  is  to  carry.  To 
make  practical  use  of  this  result  we  must  write  the  equation 

d  =  a  c$, 

where  a  is  a  constant  the  value  of  which  depends  on  the 
specific  resistance  of  the  filaments,  the  emissivity,  the  units 
employed,  and  the  temperature  at  which  the  filaments  are  to 
be  run.  It  will  be  the  same  for  all  filaments  made  by  the 
same  process.  Its  value  must  be  found  in  the  first  instance  by 
trial.  With  amyloid  carbon  at  a  temperature  of  4  watts  per 
candle-power  and  the  diameter  measured  in  mils  (10100  in.), 
and  the  current  in  amperes,  its  value  will  be  about  10.  For 
convenience  it  will,  therefore,  be  considered  to  be  10. 

For  example,  find  the  diameter  for  the  filaments  for  three 
lamps  a,  b  and  c,  to  run  at  4  watts  per  candle-power. 

(a)  to  be  100  volts,    8  c.p., 

(b)  „      100     „      16     „ 

(c)  „        60     „      32     „ 

First  calculate  the  current  which  the  lamps  will  take. 

Lamp  (a)  is  to  take  32  watts,  and  therefore  0-32  ampere, 
,,     (b)         „          64  „  „  0-64        „ 

„     (c)         „        128  „  „  2-13 

therefore  for  (a)  d  =  ac% 

=  10  x  0-468, 
d  =  4-7  mils. 

„  (b)  d  =  10x0-742, 

(1  =  7-4:  mils. 

„  (c)  d-lOx  1-655, 

t/  =  16-5  mils. 

Having  thus  found  the  diameters,  the  next  thing  to  do  is 
to  find  what  length  the  filaments  must  be. 

As  the  surface  of  the  filaments  must  be  proportional  to  the 
watts,  and  therefore  to  the  candle-powers,  and  is  also  pro- 
portional to  the  length  multiplied  by  the  diameter,  we  have 

Ixdcc  c.p., 


72  THE   INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


or  l  =  l,L, 

d 

where  b  is  a  constant,  the  value  of  which  must  be  found  in  the 
first  instance  by  trial.  In  the  present  case  we  will  suppose 
its  value  to  be  2,  using  the  same  units  as  before  —  diameter 
measured  in  mils  and  length  in  inches. 

To  find  the  lengths  for  the  above  filaments  a,  b  and  c,  we  have 

(a)  Z  =  2x  A  =  3-4  in., 

(b)  Z  =  2x—  =4-33  in., 

(c)  1  =  2  x  —  =3-88  in. 

16-5 

The  sizes  of  the  filaments  are,  therefore,  for 

(a)  4-7  mils  diameter  and  3'4in.  long, 

(b)  7-4         „  „  4-33      „ 

(c)  16-5         „  „  3-88      „ 

in  order  to  give  the  required  candle-power  at  the  volts  and 
temperature  required. 

By  carefully  measuring  the  diameter  and  length  it  is  thus 
easy  to  obtain  lamps  very  closely  alike. 

There  is,  however,  one  trouble  to  be  guarded  against  with 
unflashed  filaments.  Unless  they  have  been  carbonised  at  a 
very  high  temperature  they  are  apt  to  fall  in  resistance,  and 
will  consequently  run  too  bright.  To  obviate  this  they  must 
be  run  during  pumping  considerably  brighter  than  they  will 
afterwards  have  to  run.  This  can  very  easily  be  done,  and  it 
makes  the  filaments  settle  down  to  their  permanent  resistance, 
which  is,  of  course,  that  which  is  calculated  upon  in  deter- 
mining the  size. 

Were  it  not  for  the  superior  lasting  power  of  deposited 
carbon  the  process  of  flashing  would  not  be  resorted  to  at  all, 
as  lamps  come  out  much  more  uniform  without  being  flashed. 

Now  filaments  can,  of  course,  be  of  any  other  section  than 
circular.  Let  us  consider  how  the  sizes  of  some  other  shapes 
can  be  calculated. 


SIZES   OF   FILAMENTS   (UNFLASHED).  73 

Square  Filaments. 

Suppose  the  filaments  are  square  in  section. 
Let  the  side  of  the  square  =  o- ;  then,  as  before,  c2r  is  pro- 
portional to  the  surface.     The  surface  is  proportional  to  cr  ; 

.*.  c2roc  GP 

and  r  oc  —  ; 

c2 
•       x<r, 

o-2 

or  c  oc  <r$, 

and  cr  oc  c», 

or  o-  =  ac$ 

and  the  length  I  =  b  -  —  • 

The  constants  a  and  b  have,  of  course,  different  values  from 
those  hi  the  formulae  for  circular  filaments. 
With  filaments  made  of  the  same  material, 

a  =  8-5  and  b  =  1-575, 

or,  a  for  square  filaments  =  -85  a  for  circular  ones, 
and  6          „  „         =-788  a        „  „ 

Let  us  find  the  sizes,  using  square  filaments  for  the  same 
lamps  «,  fc,  and  c. 

(a)  o-=rtj, 

=  8-5  x -468, 
cr  =  4  mils, 

and  l  =  b^P  =1-575  x  -  =316  inches. 

o-  4 

In  the  same  way  for  (b) 

0-  =  6 -32  mils, 

I  =  4  inches  ; 

and  for  (c)  o-  =  14-l  mils, 

/  =  3-58  inches. 
Putting  these  results  side  by  side,  we  have — 

Circular  filaments.  Square  filanifnts. 

(a)     d  —   4-7  mils.  o-=    4  mils. 

1=   3-4  inches.  /=   3-16  inches. 

(6)     d=   7-4  mils.  0-=   6-32  mils. 

I  =   4-33  inches.  I  =   4  inches, 

(c)     rf  =  16-5  mils.  o-  =  14-l  mils. 

/=   3-88  inches.  1=   3-58  inches 


74  THE    INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 

The  side  of  the  square  in  the  square  filament  is  always  0-85 
of  the  diameter  of  the  corresponding  circular  filament. 

In  order  to  see  that  the  above  are  the  correct  corresponding 
values,  it  is  only  necessary  to  see  that  the  surfaces  and  the 
resistances  correspond  in  each  case. 

Take  the  lamp  (c). 

The  surface  of  the  round  filament  =  TT  d  £  =  3'14  x  16*5  x 
3,880  =  200,000  sq.  mils  ;  the  square  filament  =  4  a- 1  =  4  x  14-1 
x  3-58  =  200,000  sq.  mils  also. 

Now,    for    the    resistances    to    be    equal,    the    ratio    of 

(3n^ must  be  equal  in  both  cases,  i.e., 

cross    section 


_  must  =  — , 


=  0-018, 


Trr2     3-14  (8-25) 
and  =  J         =  0. 


Flat  Filaments. 

Filaments  which  are  made  from  sheets  of  any  material  are 
often  not  square  in  section,  but  are  wider  than  they  are  thick. 

In  this  case  there  are  three  variables  —  the  length,  the  width, 
and  the  thickness.  The  result  is  that  there  are  any  number 
of  different  dimensions  which  may  be  given  to  such  filaments, 
within  certain  limits.  Theoretically,  the  only  limit  is  in  the 
length.  The  greatest  length  is  that  when  the  section  becomes 
a  square,  there  being  no  limit  to  the  width  or  thickness.  In 
practice,  however,  there  are,  of  course,  limits  to  all  these 
dimensions. 

It  may  be  decided  to  use  filaments  of  a  certain  length,  and 
to  adjust  the  width  and  thickness  accordingly.  It  is  usually, 
however,  the  thickness  which  is  fixed,  and  the  length  and 
width  which  have  to  be  proportioned  to  suit.  If  the  filaments 
are  cut  or  punched  from  thin  sheets  of  material,  the  thickness 
of  the  sheets  determines  what  the  width  and  length  of  the 
filaments  will  have  to  be. 

First  of  all,  let  us  suppose  that  the  length  is  to  be  fixed, 
and  it  is,  therefore,  required  to  find  what  the  width  and  thick- 
ness must  be. 


SIZES    OF   FILAMENTS    (UNFLASHED).  75 

Let  t  =  the  thickness  of  the  filament, 

w  =  the  width  of  the  filament, 
h  =  the  circumference  of  the  filament, 
I  =  the  length  of  the  filament  ; 

then  7i  =  2  («+ir), 

h 


and  iv  =  --t. 

As  before,  the  surface  must  be  proportioned  to^the  power 
or,  hi  cc  c2r, 

and  r  a  _  ; 

tw 

.',,     c2/ 

,  hi  cc  —  , 

/w 

or,  ft  /  w  oc  c2, 

or,  htw  =  a  c2, 

where  a  is  a  constant,  as  before. 

Substituting  —  -  w  for  /,  we  get 

2 


or, 


4         h    ). 


_  . 
4  2 

for  filaments  of  the  same  material  as  before, 
a  =  2,460. 

Let  us  find  the  width  and  thickness  for  filaments  forjthe 
lamps  a,  b,  and  c,  as  before,  and  let  the  length  of  each  fila- 
ment be  3  inches  =  3,000  mils. 

First  of  all  it  is  necessary  to  find  h. 

,  _  surface  of  filament. 
length  of  filament. 

Now  the  surface  is  proportional  to  the  watts,  and  therefore 
to  the  candle-power,  and  for  a  given  quality  of  carbon  the 
surface  per  candle-power  will  always  be  the  same.  The 
surface  per  candle-power  for  the  particular  carbon  of  the  lamps 
a,  bt  and  c  will  be  seen  to  be  6,300  square  mils. 


76  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

Consequently  for  lamp  (a) 

surface  =  8  x  6,300  =  50,400  square  mils  ; 

.,  50400 

therefore,  h  =  =  16-8  mils, 


//16-82     4  x  2460  x  -82*\ 

,        ,  16-8     V  V    4  16-8      ~) 

and  we  have     u-  =  —  j-  ±  —  —  •=  — 

from  which  we  find  that  w  =  4-2  ±  1*68 

or  w  =  5-82  or  2-58  mils. 

Similarly  for  lamp  (b)  w  =  14-75  or  2-05  mils, 
and  for  lamp  (c)  w  =  27-6  or  6  mils. 

Of  these  two  values  given  by  the  equation,  one  is  the  thick- 
ness and  the  other  is  the  width  of  the  filament. 

Secondly,  let  us  suppose  that  the  thickness  is  fixed,  and  it 
is,  therefore,  necessary  to  find  the  width  and  the  length. 

We  have  from  above     h  t  w  =  a  c2 
and  h  =  2  (*  +  «•), 

/  /a  c2     t2\      t 
from  which  we  get  w  =     /  f--   +       - 


a  —  2,460,  as  before. 

Having  found  the  width,  the  length  can  be  found,  as  in  the 
•case  of  circular  or  square  filaments.     We  have 
I  (t  +  w)  oc  c.p., 

C.T). 

or  l  =  b  —  -' 

t  +  w 

6-8-15. 

Let  us  find  the  width  and  length  for  the  lamps  a,  b,  and  c 
as  before,  supposing  that  the  thickness  of  the  filaments  is  to 
be  8  mils. 

We  get  for  (a)  width  =     5-15  mils,  length  =  3-lin. 

(b)  „       =  11-5      „         „        =  3-48in. 

(c)  „       =  41-7      „         „        =  2-26in. 

Hollow  Circular  Filaments. 

Circular  filaments  are  sometimes  made  hollow.  Such  fila- 
ments have  been  supposed  to  be  more  efficient  than  solid  ones, 
because  there  is  no  power  being  spent  in  the  hollow  centre, 
the  power  being  all  used  in  heating  the  surface.  This  is,  of 


SIZES   OF   FILAMENTS   (UNFLASHED).  IT 

course,  a  fallacy.  Two  filaments  with  the  same  extent  of 
radiating  surface,  the  one  solid  and  the  other  hollow,  will 
require  exactly  the  same  amount  of  power  to  maintain  them 
at  the  same  temperature.  If  they  are  the  same  length,  the 
only  difference  will  be  that  the  hollow  one  will  have  a  higher 
resistance,  and  will,  therefore,  take  a  greater  voltage  and  less 
current  than  the  other,  but  the  product  of  the  volts  and 
amperes  (watts)  will  be  the  same  for  each.  The  effect  of 
making  filaments  hollow  can  be  best  seen  by  finding  the  sizes 
for  the  lamps  a,  b,  and  c,  as  before. 
Let  d  =  the  outer  diameter, 

8  =  the  inner        ,, 

d  =  n  8. 

As  before,        surface  oc  c2  r, 
or  d  I  oc  c2  r, 

I 
and  r  oc  ^^  ; 

C2i 

therefore,  dl  ac  .  ' 


c2 
or  dec  J2T782'  or  d  (^  ""  82)  a  c' 

Substituting  -  for  8,  and  simplifying,  we  get 
n 


the  constant  a  having  the  same  value  as  in  the  formula  for 
solid  circular  filaments.  The  formula  for  length  (and  the 
constant  b)  is  the  same  as  for  solid  filaments, 


We  have  then  only  to  take  the  values  found  for  circular  solid 
filaments,  and  multiply  by 

«2   \i 


It  is,  however,  more  convenient  in  the  first  instance  to  think 
of  a  certain  ratio  of-  the  diameter  to  the  actual  thickness  of 
the  wall  of  the  filament  than  of  the  ratio  of  the  diameter  to 
the  internal  diameter. 

Therefore,  let  t  =  thickness  of  the  wall  of  the  filament,  and 
let  7?i  =  the  ratio  of  the  diameter  to  the  thickness. 


78 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


=  n8  =  m  t, 
d-8 


Then 

and 

z 

Consequently,  n  =  — -^ 

Now,  in  the  lamps  a,  b,  and  c,  let  the  thickness  of  the  wall  of 
the  filaments  be  one-tenth  of  the  diameter. 
Then  wi  =  10 

and  n  =  —  =  1'25. 

We  therefore  get,  taking  the  values  found  for  circular  solid 
filaments,  and  multiplying  them  by  (-* j   =1-41, 


I 


5-28  mils. 
8-39     „ 
18-65 


0-66  mils. 
1-04     „ 
2-33 


2-43*»iis.  inch. 
3-06  -TT 
2-75 


d 

(a)  6-6    mils. 

(b)  10-47     „ 

(c)  23-3       „ 

We  have  now  seen  how  the  sizes  of  filaments  may  be 
obtained  for  the  most  usual  forms  of  cross  section.  The 
results  found  for  the  lamps  a,  b,  and  c  are  here  tabulated 
together.  It  will  be  found  that  the  resistance  and  the  surface 
of  all  the  varieties  for  each  of  the  lamps  a,  b,  and  c  are  the 
same  in  each  case,  and  therefore  the  lamps  will  be  of  the 
same  volts,  amperes,  and  candle-power  in  each  case. 


Form  of  Section. 

Lamp  (a) 
100  Volts, 
8  Candles, 
32  Watts. 

Lamp  (b) 
100  Volts, 
16  Candles, 
64  Watts 

Lamp  (c) 
60  Volts, 
32  Candles, 
128  Watts. 

Circular     -! 

diameter    ...(mils) 
length  (inches) 

4-7 
3-4 

7'4 
4-33 

16-5 
3-88 

side     (mils) 

4-0 
316 

6-32 
4-0 

14-1 
3-58 

length  (inches) 

Rectangular  when    J 
length  is  fixed  ...  1 

length  (inches) 
thickness    ...(mils) 
width  (mils) 

3-0 
2-58 
5-82 

3-0 
2-05 
14-75 

3-0 
6-0 
27-6 

Rectangular  when    J 
thickness  is  fixed  1 

thickness   ...(mils) 
width     (mils) 
length  (inches) 

3-C 
5-15 
3'1 

3-0 
11-5 
3-48 

3-0 
41-7 
2-26 

Hollow    circular  f 
d 

outside  diam.(niils) 
inside  diam.  (mils) 

6-6 

5-28 

10-47 
839 

23-3 
18-65 

when  7  is  fixed  ;..  | 

length  (inches) 

2-43 

3-06 

2-75 

*                   ( 

thickness    ..  (mils) 

0-66 

1-04 

2-33 

SIZES    OF    FILAMENTS    (UNFLASHED).  79 

The  solid  circular  form  gives  the  longest  filament,  the  ratio 
of  section  to  circumference  being  then  greater  than  with  any 
other  form.  The  tubular  form  gives  the  next  longest,  and 
may  give  any  length,  from  that  of  the  circular  solid  down- 
wards. The  rectangular  section  gives  any  length,  from  that 
where  the  section  is  a  square  downwards. 

The  law  that  the  power  spent  in  a  filament  in  order  to 
maintain  it  at  a  certain  temperature  must  be  proportional  to 
the  extent  of  radiating  surface  is  only  strictly  true  when  the 
filaments  are  straight,  not  bent  into  the  form  of  a  horseshoe, 
or  other  shape.  That  is  to  say,  it  is  only  true  when  the 
filament  does  not  radiate  to  and  from  itself.  ^Yith  circular 
filaments  of  small  diameter,  bent  into  the  horseshoe  form, 
with  a  distance  between  the  sides  of  perhaps  a  hundred  times 
the  diameter,  the  error  is  inappreciable.  When,  however,  the 
diameter  of  the  filament  is  much  greater  than  a  hundredth 
part  of  the  distance  between  the  two  legs  there  may  be  a 
serious  error  in  using  these  formulae.  The  error  will  be  most 
serious  in  the  case  of  flat  filaments.  If  a  flat  filament  be 
selected,  according  to  the  formula  given,  and  be  mounted  in 
the  lamp  so  that  the  thin  edges  are  towards  each  other,  it 
will  come  out  all  right.  If,  however,  it  be  mounted  with  its 
broad  sides  facing  each  other,  it  will  run  too  bright. 

The  effect  of  flashing  upon  the  sizes  of  the  filaments  will 
now  be  considered. 


CHAPTER  VII. 


SIZES  OF  FILAMENTS  (FLASHED). 

THE  effect  of  flashing  is  threefold,  as  already  explained.  It 
alters  the  emissivity,  the  resistance  and  the  thickness  of  the 
filament.  It  diminishes  the  resistance,  and,  usually,  the  emis- 
sivity, and  increases  the  thickness.  The  effect  of  increasing 
the  thickness  is  opposite  to  that  of  diminishing  the  emissivity- 
That  is  to  say,  increase  of  thickness  means  increase  of 
candle-power,  and  decrease  of  emissivity  means  decrease  of 
candle-power.  The  reduction  in  resistance  and  the  increase 
in  thickness  continue  to  proceed  as  long  as  the  process  is 
going  on.  The  emissivity,  however,  is  changed  almost  entirely 
within  the  first  few  seconds,  after  which  it  remains  practically 
constant. 

What  the  lamp-maker  wants  to  know  is  what  sized  filament 
he  must  take  in  order  to  produce  such  and  such  a  lamp.  If 
he  is  making  unflashed  filaments  the  matter  is  very  simple, 
and  the  dimensions  are  readily  calculated,  as  already  explained. 
If,  on  the  contrary,  he  wishes  to  flash  his  filaments,  the  con- 
ditions are  complicated  by  the  three  separate  effects  just  men- 
tioned, which  we  will  now  deal  with. 

A  formula  for  calculating  diameters  for  flashed  filaments,  on 
the  same  lines  as  those  already  given  for  unflashed  ones, 
might  here  be  given.  Owing,  however,  to  the  necessity  for 
the  introduction  of  constants,  on  account  of  the  difference  in 
specific  resistance  of  the  filaments  and  the  deposited  carbon, 
it  becomes  too  cumbrous  for  practical  use,  and  is,  therefore, 
omitted.  Instead  of  using  the  formula,  a  number  of  different 
cases  can  be  worked  out  separately,  and  the  results  plotted  out 
in  curves,  from  which  any  other  sizes  can  be  easily  and  quickly 
obtained.  Such  curves  are  shown  in  Figs.  24  and  25. 

a 


82 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


DIAMETER    OF    FILAMENT  BEFORE    FLASHING  ;  MILS. 


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84  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

Before  commenting  upon  the  curves,  the  method  by  which 
they  were  calculated  will  be  explained.  The  filaments  them- 
selves are  of  the  same  quality  (i.e.,  specific  resistance  and 
emissivity)  as  those  of  the  three  lamps  #,  b,  c,  sizes  for 
several  shapes  of  which  have  been  already  given.  This  par- 
ticular quality,  as  already  mentioned,  is  about  the  usual  one 
obtained  with  filaments  made  from  amyloid.  For  unflashed 
circular  filaments  it  was  shown  (p.  71)  that  d  =  ac%,  and  that 
with  this  carbon  the  value  of  a  is  10. 

In  Figs.  24  and  25  the  dotted  curve  A  gives  diameters  for 
currents  up  to  3  amperes  for  this  quality  of  unflashed  carbon. 

We  will  now  suppose  that  filaments  of  this  same  carbon  are 
flashed,  and  that  the  flashing  be  done  by  the  pentane  process. 
The  process  is  arranged  as  to  the  amount  of  pentane  vapour 
in  the  flashing  chambers,  the  degree  of  exhaustion,  and  the 
temperature  to  which  the  filament  is  heated,  so  that  the 
deposit  of  carbon  proceeds  at  the  rate  of  0-05  mil  in  thickness 
in  10  seconds ;  that  is  to  say,  the  diameter  of  the  filament  is 
increased  by  twice  that  amount,  or  0*1  mil  in  10  seconds. 
This  is  a  convenient  rate  in  practice,  and  one  that  can  be 
easily  obtained  by  most  vacuum  processes. 

From  the  dimensions  found  for  the  unflashed  lamps  a,  b 
and  c  in  the  last  chapter,  the  specific  resistance  of  the  carbon 
can  be  calculated,  For  convenience,  we  will  take  the  re- 
sistance of  one  inch  length  of  one  square  mil  section.  From 
lamp  b  we  see  that  a  filament  7*4  mils  in  diameter  and  4*33in. 
long  has  a  resistance  of  156  ohms  ;  consequently,  one  square 
mil  lin.  long  has  a  resistance  of  1,560  ohms. 

The  deposited  carbon  produced  as  described  has  a  specific 
resistance  of  about  one-tenth  of  that  amount,  or  one  square 
mil  lin.  long  equals  156  ohms.  The  emissivity  of  this  de- 
posited carbon  is  less  than  that  of  the  carbon  of  the  filament 
itself  in  the  proportion  of  14  to  10 ;  that  is  to  say,  if  an  un- 
flashed filament  at  the  temperature  of  4  watts  per  candle- 
power  gives  a  light  of  14  candles,  a  filament  of  exactly  the 
same  extent  of  surface  when  flashed  will  give  only  10  candles 
at  the  same  temperature. 

With  these  data  then,  assuming  any  dimensions  we  like  for 
the  filament  and  any  duration  for  the  flashing  process,  we  can 
calculate  the  current  which  it  will  take  when  flashed  to  main- 


SIZES    OF    FILAMENTS    (FLASHED).  85 

tain  it  at  the  required  temperature  (in  the  present  case  always 
that  of  4  watts  per  candle-power),  and  we  can  then  plot  the 
results  in  curves. 

For  example,  take  the  case  of  a  filament  10  mils  in  diameter. 
We  want  to  find  what  current  it  will  take,  when  flashed  for,  say, 
80  seconds,  to  maintain  it  at  the  temperature  of  4  watts  per 
candle-power.  Suppose  it  be  Sin.  long,  what  is  its  resist- 
ance ?  We  know  that  lin.  length  one  square  mil  in  section  of 
this  carbon  =  1,560  ohms.  Therefore,  we  get  the  resistance 

of  this  filament  =  1>5?>*5  =99-3  ohms.  What  candle-power 

will  it  be  at  the  temperature  of  4  watts  per  candle-power? 
We  know  (from  the  sizes  on  p.  71)  that  6,300  square  mils  of 
surface  of  this  carbon  gives  1  c.  p.  at  that  temperature  ;  con- 

10  XTTX  5,000 

sequently.  the  candle-power  of  this  filament  =  --  (TgOO  — 

=  25.  Suppose  now  that  it  is  flashed  for  30  seconds.  It  will 
then  have  a  coating  of  deposited  carbon  0-15  mil  thick.  Its 
diameter  will  be  increased  by  twice  that  amount  and,  therefore, 
will  be  10-3  mils.  The  resistance  of  the  deposited  carbon  is 
that  of  a  tube  10  mils  diameter  inside  and  10-3  mils  outside 
and  Sin.  long.  The  sectional  area  =  IT  [  (r  +  t)2  -  r2)  ]  =  4-79 
square  mils.  The  resistance  of  the  deposited  tube,  then,  is 
that  of  4-79  square  mils  Sin.  long,  and  is  therefore  163  ohms 
(one  square  mil  lin.  long  =  156  ohms). 

The  flashed  filament  is  therefore  made  up  of  the  filament 
proper  of  99*3  ohms,  over  which  is  a  tube  of  flashed  carbon  of 
163  ohms.  The  resultant  resistance  of  the  filament  is  conse- 
quently 61  '7  ohms. 

Now  the  candle-power  of  the  filament  before  flashing  would 
have  been  25,  but  by  flashing  its  emissivity  is  reduced  in  the 
ratio  of  14  to  10.  But  it  has  also  been  thickened  in  the  ratio 
of  103  to  100 

Therefore  its  candle-power  will  now  be 


and  the  power  required  will  be  18*4  x  4  =  73*6  watts. 

XT  o        "'         73'6        -|    i  rvo 

Now  c2  =  ~-=—  =1-193; 

therefore,  c  =  1-092  amperes. 


86  THE    INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 

Thus  a  10-mil  filament  flashed  for  30  seconds  takes  a  current 
of  1/092  amperes,  and  the  filament  becomes  10'3  mils  diameter. 
The  same  result  is,  of  course,  arrived  at  whatever  the  length 
of  the  filament  is  assumed  to  be,  the  length,  as  previously 
explained,  having  nothing  to  do  with  the  current  strength. 

By  working  out  a  number  of  cases  in  this  way,  curves  such 
as  those  in  Figs.  24  and  25  can  be  drawn. 

Having  obtained  the  curves,  they  are  useful  in  enabling  us 
to  do  the  reverse  process  at  a  glance.  For  instance,  a  par- 
ticular lamp  is  to  be  made,  and  its  current  must  be  so-and-so. 
The  curves  show  at  once  what  diameters  of  filament  may  be 
taken,  and  how  much  they  must  be  flashed  in  order  to  produce 
the  result. 

If  it  is  desired  to  give  the  filaments  a  definite  thickness 
of  deposit,  there  is  one  diameter  alone  which  will  answer  the 
purpose.  Suppose  that  it  is  desired  to  give  a  coating  OS  mil 
thick.  It  takes  60  seconds  to  do  this.  Take  the  case  of  lamp 
b,  a  100-volt  16-c.p.  0-64-ampere  lamp.  We  find  on  the  60 
seconds  curve  (Fig.  24)  that  we  must  take  a  filament  5-3 
mils  diameter  to  give  this  current.  During  the  process  of 
flashing  it  will  become  thickened  to  5-9  mils,  a  fact  which 
must  be  remembered  in  estimating  its  length.  The  length  to 

O  1") 

be  taken  is  given  as  before  by  the  formula  I  =  b  -j-i  &  being  the 

ct 

finished  diameter,  the  value  of  b  in  the  case  of  the  flashed 
surface  being  2-8  (i.e.,  the  constant  for  the  unflashed  surface 
multiplied  by  14/10,  the  ratio  of  the  emissivities)  ; 

1  fi 

therefore  I  =  2-8  x  —  =  7'6in. 

5-9 

7*6in.  of  5-3  mils  diameter  filament  must  then  be  flashed  for 
60  seconds,  and  it  will  then  be  of  the  right  resistance  and 
surface. 

This,  therefore,  shows  another  way  by  which  filaments  may 
be  flashed  to  the  required  resistance.  No  instruments  are 
needed  except  a  watch  to  show  the  time  of  flashing.  Start 
with  the  proper  size  of  filament  shown  by  the  curve,  and 
flash  it  for  the  required  length  of  time,  and  it  is  brought  to 
the  required  resistance.  Such  a  method,  however,  would  not 
work  well  in  practice  owing  to  the  difficulty  of  exactly  repro- 
ducing to  conditions  each  time.  If,  however,  the  right  size  of 


SIZES   OF    FILAMENTS    (FLASHED).  87 

filaments,  as  shown  by  the  curve,  are  taken  and  flashed  to  their 
proper  resistance  hot,  the  time  of  flashing  will  be  60  seconds 
on  the  average. 

It  will  be  seen  by  examining  the  curves  that  there  is  a  wide 
range  of  possible  sizes  for  the  filament,  according  to  the 
amount  of  flashing  it  receives.  For  the  same  lamp  taking 
0'64  ampere,  a  filament  3*7  mils  in  diameter  flashed  for  two 
minutes  gives  the  required  result,  or  one  of  7-65  mils  flashed 
for  only  ten  seconds  will  do  equally  well. 

It  has,  however,  been  pointed  out  in  practice  that  30 
seconds  is  about  as  long  a  time  as  can  be  allowed  for  flashing. 
By  taking  the  sizes  given  by  the  30- seconds  curve  we  get 
always,  for  any  current,  the  diameter  which  will  take  30 
seconds  to  flash  to  its  required  resistance. 

On  examining  the  curves,  one  thing  that  immediately 
attracts  attention  is  that  the  curves  for  the  flashed  filaments 
D,  E,  F,  G,  H,  J,  cross  the  curve  for  the  unflashed  ones  A, 
at  some  point.  This  is  on  account  of  the  variation  in  the 
equivalent  specific  resistance  of  the  flashed  filaments. 

The  topmost  heavy  curve  B  shows  what  the  result  would 
be  if  the  filaments  could  be  flashed  so  as  to  give  the  less  emis- 
sivity  of  the  deposited  carbon,  but  without  changing  the  resis- 
tance. In  other  words,  it  is  the  curve  for  a  carbon  of  the 
specific  resistance  of  the  unflashed,  but  with  the  emissivity  of 
the  deposited  variety.  It  is  introduced  in  order  to  show  the 
limit  of  diameter  for  any  current  beyond  which  it  is  impossible 
to  go  under  any  conditions  with  these  two  qualities  of  carbon. 
In  the  other  direction  the  limit  of  the  diameter  is  zero ;  that 
is  to  say,  the  amyloid  filament  vanishes  altogether,  deposited 
carbon  alone  remaining. 

Thus  the  diameter  selected  for  a  1-25-ampere  filament  at 
the  temperature  of  4  watts  per  candle-power  may  be  anything 
from  12-9  mils  down  to  nothing,  according  to  the  time  of  flash- 
ing. If  unflashed  carbon  is  used,  the  diameter,  as  shown  by  the 
dotted  curve  A,  will  be  11*6  mils  ;  if  flashed  for  10  seconds,  it 
will  be  12-3  mils  ;  for  20  seconds,  11*7  mils  ;  for  30  seconds, 
11*15  mils  ;  for  one  minute,  9-7  mils  ;  for  two  minutes,  7-5 
mils.  The  limit  of  zero  diameter  is  reached  (supposing  such 
a  thing  possible)  at  the  ten-minutes  curve.  It  must  be  re- 
membered that  the  diameters  shown  by  the  curves  on  Fig.  24 


58  THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

are  the  diameters  of  the  filaments  before  flashing,  and  that  the 
actual  finished  diameters  are  greater  according  to  the  length 
of  time  they  are  flashed  by  one-tenth  of  a  mil  for  every  10 
seconds.  In  the  limit  when  the  diameter  of  the  unflashed 
carbon  becomes  zero  the  actual  diameter  is  0  +  6  mils,  6  mils 
being  the  increase  in  diameter  produced  in  ten  minutes.  In 
other  words,  the  filament  is  now  entirely  of  deposited  carbon, 
and  6  mils  in  diameter.  Speaking,  then,  of  the  final  or  flashed 
diameter,  the  limits  for  1-25  amperes  are  12-9  mils  and  6  mils, 
6  mils  being  the  diameter  of  a  solid  filament  of  deposited 
carbon  alone.  The  lower  heavy  curve  C  gives  the  values  for 
such  material.  It  of  course  follows  the  d  =  a  c$  law,  the  value 
of  the  constant  a  being  5-17.* 

In  order  to  make  the  matter  as  clear  as  possible  the  second 
sheet  of  curves  (Fig.  25)  is  given,  showing  the  final  or  flashed 
diameters.  It  will  be  noticed  that  all  the  sizes  lie  between  the 
limits  of  the  heavy  curves  B  and  C.  It  is,  however,  the 
diameters  of  the  filaments  before  flashing  that  the  lamp- 
maker  requires  in  the  first  instance. 

Looking  at  the  curves  (Fig.  25)  we  see  for  any  current 
within  what  limits  the  final  diameter  of  the  filament  for  that 
current  must  lie.  Thus,  for  two  amperes  the  final  diameter 
will  lie  between  8-2  and  17-7  mils.  Theoretically,  any  dia- 
meter between  these  limits  may  be  made  to  do  for  two 
amperes,  according  to  the  length  of  time  of  flashing,  or  no 

*  The  value  of  the  constant  may  be  found  by  taking  any  particular  case 
and  working  it  out,  thus 


Suppose  the  filament  of   a  solid  deposited  carbon  to   be  Sin.  long  and 
10  mils  diameter,  its  sectional  area  =  78'54  square  mils  and  its  resistance, 

therefore,  is  -j     *    =  9'93  ohms. 

' 


Its  surf  ace  =  157,000  sq.  mils. 

The  surface  per  candle-power  of  the  deposited  carbon 

=  6,300  x  15  =  8,820  sq.  mils; 


therefore  its  candle-power  =  zfpijrr  =  17'8f  and  the  watts  =  17'8  x  4  =  71  '2 
8,820 

therefore  c2  =  !  =  <S!  =  7'17' 

and  cS  =  l-93; 

consequently  a=  -     =5*17. 

1*89 


SIZES    OF    FILAMENTS    (FLASHED).  89 

flashing  at  all  if  the  filament  be  15-9  mils,  as  shown  by  the 
dotted  curve  A. 

In  actual  practice  the  limits  are  somewhat  more  restricted, 
on  account  of  the  difficulty  of  making  or  handling  very  thin 
filaments  or  of  flashing  properly  in  less  than  10  seconds.  The 
practical  limits  for  this  current  may,  however,  be  considered  as 
10  mils  and  17  mils  finished  diameter,  still  a  very  wide  range, 
according  as  to  whether  the  flashing  be  done  for  10  seconds 
or  up  to  five  or  six  minutes.  This  means  that  the  filament 
before  flashing  will  have  a  range  of  from  4  or  5  mils  to  17  mils 
in  diameter.  If  the  time  of  flashing  is  to  be  restricted  to 
30  seconds,  then  16-05  mils  will  be  the  finished  diameter  and 
15-75  the  diameter  before  flashing. 

The  fact  that  flashing  a  filament  may  reduce  as  well  as 
increase  the  current  strength  necessary  to  raise  it  to  a  given 
temperature  is  one  which  the  Author  believes  to  be  not 
generally  realised.  Looking  at  the  curves  for  finished 
diameters,  it  is  seen  that  an  unflashed  filament  of  14  mils 
diameter  takes  about  1-66  amperes.  A  filament  of  this 
diameter  which  has  been  flashed  for  10  seconds  takes  only 
1-49  ampere.  One  which  has  been  flashed  for  20  seconds 
takes  1-58  ampere,  while  one  flashed  for  30  seconds  takes 
1-66  ampere,  the  same  as  the  unflashed  filament  of  that 
diameter.  If  flashed  for  a  longer  time  than  30  seconds,  it 
takes  more  current  than  the  unflashed  one. 

^Yllen  the  diameter  and  current  are  the  same  for  un- 
flashed and  flashed,  as  in  this  case  for  fourteen  mils  and 
thirty  seconds,  the  lengths  of  the  filament  to  produce  the 
same  volts  and  candle-power  will  be  in  the  proportion  of  ten 
to  fourteen,  the  length  of  the  flashed  filament  being  greater 
in  inverse  proportion  to  the  alteration  in  ernissivity. 

Looking  at  the  curves  showing  the  diameters  before  flashing 
we  see  that  an  unflashed  filament  of  16-8  mils  diameter  takes 
2-18  amperes.  If  this  same  filament  be  flashed  for  10  seconds, 
it  takes  only  1-97  ampere,  if  it  be  flashed  for  20  seconds  it 
takes  2-07  amperes,  and  if  for  30  seconds  it  takes  2-18 
amperes,  the  same  as  it  did  before  being  flashed.  Its  diameter 
is,  of  course,  increased  by  0-3  mil,  and  is  thus  17-1  mils. 

Here,  again,  let  there  be  no  misunderstanding  about  effi- 
ciency. The  filament  which  is  flashed  for  a  short  time  and 


90  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

which  takes  less  current  and  fewer  volts  than  it  did  before 
flashing  to  maintain  it  at  a  certain  temperature,  is  no  more 
efficient  than  before  flashing.  The  power  required  to  produce 
the  temperature  is  less.  In  precisely  the  same  proportion 
is  the  candle-power  also  less. 

When  filaments  are  flashed  to  a  certain  resistance  measured 
cold,  it  has  already  been  pointed  out,  that  owing  to  the 
widely  diiferent  temperature  coefficients  of  the  filament  and 
the  deposited  carbon,  it  is  essential  that  the  ratio  of  the 
two  amounts  of  these  carbons  present  in  the  filament  be 
known.  In  a  factory  where  the  cold  measurement  is  adopted 
it  should  be  a  rule  always  to  flash  the  filaments  so  that 
there  is  a  fixed  ratio  between  the  sectional  area  of  the  fila- 
ment and  that  of  the  deposit,  that  is  to  say,  between  their 
resistances. 

It  has  been  mentioned  that  the  general  formula  for  dia- 
meters of  flashed  filaments  is  too  cumbrous  for  ordinary  use, 
owing  to  the  difference  in  the  specific  resistance  of  the  two 
kinds  of  carbon  having  to  be  taken  into  consideration.  When, 
however,  there  is  a  fixed  ratio  between  the  quantities  of  the 
two  kinds  of  carbon  in  the  filament,  the  formula  becomes 
again  of  the  simple  form,  d  =  a  c§ ;  because  in  such  a  case 
we  may  consider  the  flashed  filaments  as  having  a  definite 
specific  resistance.  In  order,  then,  to  find  the  right  diameters 
to  use,  so  that  there  shall  always  be  a  fixed  ratio  between  the 
resistance  hot  and  the  resistance  cold,  it  is  only  necessary  to 
find  the  value  of  the  constant  a  for  whatever  ratio  is  required, 
and  the  formula  gives  the  required  result.  With  the  same 
qualities  of  filament  and  deposited  carbon,  then,  let  us  find 
the  value  of  a,  so  that  the  cold  resistance  may  be  for  all 
diameters  equal  to  twice  the  hot  resistance. 

It  has  been  mentioned  (p.  62)  that  the  ratio  of  the  cold 
resistance  to  the  hot  in  the  case  of  the  filaments  is  as  1*5  to  1, 
and  in  the  case  of  the  deposited  carbon  as  2-5  to  1.  We  want 
to  combine  them  in  such  a  proportion  that  the  ratio  shall  be 
as  2  to  1. 

With  these  ratios,  if  the  filament  be  100  ohms  hot,  the 
deposit  must  be  60  ohms  hot.  The  resultant  resistance  hot 

will  then  be  10x  60  =  37-5  ohms. 


SIZES    OF    FILAMENTS    (FLASHED).  91 

The  cold  resistance  of  the  filament  will  be  100x1-5  =  150 
ohms,  and  that  of  the  deposit  will  be  60  x  2-5  =  150  ohms. 
The  combined  resistance  cold  will  therefore  be  75  ohms, 

2x37-5  =  75. 

In  order,  then,  for  the  resistance  cold  to  be  twice  the  hot 
resistance,  the  hot  resistance  of  the  deposit  must  equal  0-6  of 
the  hot  resistance  of  the  filament. 

Now  the  specific  resistance  of  the  filament  is  ten  times  that 
of  the  deposited  carbon.  Consequently,  the  sectional  area  of 
the  filament  must  be  10  x  0*6  =  6  times  that  of  the  deposit. 

Let  r  =  radius  of  the  filament, 

t  =  thickness  of  the  deposit, 
section  of  filament  =  TT  r2, 

section  of  deposit  =ir[  (r  +  «)2  -  r2]  ; 

therefore     6  TT  [  (r  +  t)2  -  r2]  =  TT  r2  ; 
from  which  we  get     t  =  0-081  r,     or  r  =  12-37*; 
£  =  0-0405d,  or  d  =  24-74*, 
where  d  =  2r  ;  or,  if  D  =  the  finished  diameter,  t  =  0-0374  D. 

With  flashing  at  the  same  rate  as  before,  i.e.,  0-05  mil 
thickness  of  deposit  in  ten  seconds,  it  follows  that  for  every 
mil  in  diameter  the  filament  must  be  flashed  for  8*1  seconds  to 
produce  the  required  ratio. 

As  before  d  =  a  c$,  and  therefore  a  =  «~     In  order  to  find  a, 

c*« 

take  any  filament  and  find  c§.  Let  the  filament  be  10  mils 
in  diameter,  and  t  will  be  0-405  mil.  For  convenience  of 
having  round  numbers  in  the  resistance  let  the  length  be 
5-05in.  The  sectional  area  of  the  filament  equals  78-54 
square  mils,  and  the  resistance,  therefore,  is 


The  sectional  area  of  the  deposit  =  ir  (5-4052  -  52)  =  13-1  square 
mils.     And  the  resistance  of  the  deposit 

_  156  x  5-05  =  60ohms 
13'1 

The  resultant  resistance,  as  we  have  already  seen,  is  therefore 

100^0  =37.5ohms 


92  THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

Now,  the  surface  of  the  filament  =  length  x  circumference 
=  5,050  x  10-81  x  TT  =  171,500  square  mils.    One  candle-power 

14 
of  this  flashed  surface  =  6,  300  x  —  =8,820  square  mils;  con- 

sequently the  candle-power  of  this  filament 

_  171,500       9 

5< 


And  the  power  =  19-5  x  4  =  78  watts, 


therefore  c  =  1-44  ampere, 

and  c3  =  l-27, 

and  a  =  JO_  =  7-86; 

or  for  finished  diameter  a  =  1Q  t81  =  8  -46. 

1  *27 

The  curves  K  and  K'  (Figs.  24  and  25')  give  the  diameters 
before  and  after  flashing. 

Supposing,  then,  that  the  filaments  are  flashed  until  the 
resistance  measured  cold  is  brought  down  to  the  value  of 
twice  the  required  resistance  hot,  we  can  at  once  find  the 
diameter  for  any  current  from  these  curves,  and  we  can  also 
see  the  time  which  will  be  occupied  in  the  flashing  by  the 
crossing  of  the  curve  with  the  time  curves  D,  E,  F,  G,  &c. 

For  example,  suppose  we  are  flashing  to  cold  resistance  and 
wish  to  make  lamp  b  (p.  78)  100  volts  16  c.p.  0-64  ampere. 
Looking  at  the  curves  K  and  K',  we  see  that  for  this  current 
the  diameter  before  flashing  is  5-8  mils,  and  after  flashing  is 
6-3  mils.  The  length  required  will  be  given,  as  before,  by  the 
formula, 


The  resistance  hot        =  -  =  12°  =  156-25  ohms. 
c      0-64 

Consequently,  the  cold  resistance  must  be  2  x  156-25  =  812-5 
ohms.  If,  therefore,  we  take  a  filament  5-8  mils  thick  and 
7-lin.  long,  and  flash  it  to  312-5  ohms  cold,  we  get  what  we 
require. 

The  values  taken  from  the  curves  are  as  near  as  can  possibly 
be  required  in  practice,  but  if  it  is  wished  to  know  the  figures 


SIZES    OF    FILAMENTS   ^FLASHED).  93 

more  closely,  we  can  use  the  formula 

d  =  ac$,  «  =  8-46, 
from  which  d  =  6-284  mils. 

Let  us  see  if  these  values  work  out  correctly.  The  final 
diameter  is  equal  to  the  diameter  before  flashing  +  twice  the 
thickness  of  the  deposit,  and  (p.  91)  we  know  that  the  diameter 
before  flashing  must  =  24-74  times  the  thickness  of  the 
deposit  ;  consequently,  the  diameter  before  flashing 

=  x  6-284  =  5-82  mils. 


The  section  of  the  filament        =  arr2  =  26-585  square  mils. 
1  square  mil  lin.  long  =  1,560  ohms  ; 

.-.  26-585  nmT- 

or  the  resistance  of  the  filament  =  416*6  ohms. 
The  section  of  the  deposit          =  7r[(r  +  z)2  -  r2] 

=  4-431  square  mils. 
1  square  mil,  lin.  long  =156  ohms; 

/.  4-431  mil  7-1  long  =15J3.*J'1, 

or  the  resistance  of  the  deposit  =  250  ohms, 

416-6  x  250 
and  the  combined  resistance          =  416<6  +  25Q  ohms. 

Let  us  now  find  the  candle-power.     The  surface  =  TT  x  6-284 
x  7,100=  140,200  square  mils.      Therefore  the  candle-power 
_  140,200  _lfi 

8,820 
The  volts,  amperes  and  candle-power  are  therefore  correct. 

Lastly,  let  us  see  if  the  resistance  cold  is  twice  the  resist- 
ance hot. 

The  resistance  of  the  filament  hot  =  416-6  ohms,  therefore, 
cold,  it  is  416-6  x  1-5  =  625    ohms. 

The  resistance  of  the  deposit  hot      =  250    ohms,  therefore, 
cold,  it  is  250x2-5  =  625    ohms. 

The  combined  resistance  cold,  therefore,  is 
625x625 


625  +  625 

which  is  twice  the  hot  resistance,  hence  the  filament  fulfils 
all  the  conditions  required  of  it. 


94  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

Filaments  of  any  diameter  flashed  for  less  time  than  that 
indicated  by  the  curve  K,  will  have  a  ratio  of  cold  to  hot 
resistance  of  less  than  2,  and  if  flashed  for  a  longer  time 
will  have  a  greater  ratio. 

Instead  of  finding  the  value  for  the  constant  a,  in  the 
formula  d  —  a  c$  by  the  method  of  working  out  a  particular 
example,  we  will  proceed  to  consider  how  its  value  may  be 
found  for  any  case  where  the  current  and  diameter  follow 
this  law. 

The  constant  a  is  really  made  up  of  two  other  constants, 
which  we  will  call  a  and  J3.  We  have  seen  that 

c-  r  oc  d,  and  that  r  oc  — - 
Let  c2  r  =  a  d  for  unit  length  of  filament, 

and  let  r  =  -0    f°r  »  » 

a2 

then  c2  x  Ji  =  a  d, 

or  c2  =  ^3   and  d3  =  £c2, 

ft  a 

or  "<(£)*  *<§s 

•consequently,  the  constant  a •»/£? \»« 

Taking  the  unflashed  carbon  as  the  starting  point,  let  us 
find  the  values  for  a  and  /?.  Take  the  case  of  a  filament 
10  mils  in  diameter  and  lin.  long. 

The  surface  =  10  x  TT  x  1,000  =  31,416  square  mils.  We 
know  that  6,300  square  mils  =  1  c.p. 

Therefore  the  candle-power  =  — _L —  =  5, 

6,300 

;and  the  power  =  5  x  4  =  20  watts. 

..£r-*>.*. 

d        10 

The  sectional  area  of  the  filament  =  ?rr2  =  78*54  square 
mils.  We  know  that  one  square  mil  lin.  long  =  1,560  ohms  ; 

therefore  the  resistance  =   *        =  20  ohms. 
/3  =  d*r  =  2,000, 


SIZES    OF    FILAMENTS    (FLASHED).  95 


and  therefore  „  -  -  -  10, 


which  is  the  value  assigned  to  a  at  the  first  (p.  71). 

We  are  now  able  to  find  the  value  of  the  constant  a  for 
any  carbon,  if  we  know  its  relative  specific  resistance  and 
emissivity  to  that  of  this  carbon. 

Let  us  find  its  value  by  this  method  for  the  same  carbon 
flashed  so  that  its  cold  resistance  shall  be  equal  to  twice  its 
hot  resistance. 

We  know  that  by  being  flashed  its  emissivity  will  be  reduced 
in  the  proportion  of  14  to  10  ;  consequently, 

a  =  2x™  =  l-43. 

As  in  this  case  there  is  always  a  fixed  ratio  between  the 
sectional  area  of  the  deposit  and  that  of  the  filament,  we  can 
find  its  equivalent  specific  resistance. 

It  has  been  shown  that  the  resistance  of  the  deposit  must 
be  0*6  times  that  of  the  filament.  If,  therefore,  the  resist- 
ance of  the  filament  be  100  ohms,  that  of  the  deposit  will 
be  60  ohms,  and  the  combined  resistance  will  be  37*5  ohms. 
But  in  being  flashed  the  diameter  of  the  filament  is  increased 

in  the  ratio  of  ™J7  (p.  91). 

Consequently,  the  resistance  of  an  unflashed  filament  of 
the  same  diameter  and  length  as  the  flashed  one  will  be 

(1O.Q7\  2 
t^±  \  =  85-6  ohms.    Therefore  the  equivalent  specific 
Io'o7/ 

conductivity  has  been  increased  by  flashing  in  the  ratio  of 

85-6  _  o.oo  . 
87*- 

consequently,  P  =  2,000  x    -L  =  876  ; 


therefore,  a  -  J/f  -  ;J  ™  -  8-46, 

which  is  the  value  found  on  page  92. 

In  the  same  way,  by  first  finding  the  values  for  a  and  f3, 
the  value  of  a  can  be  found  for  any  quality  of  carbon,  or  for 
flashed  carbon  which  has  a  definite  ratio  between  the  sectional 
area  of  the  filament  and  that  of  the  deposit. 


96 


THE    INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 


The  following  values  for  a,  for  filaments  flashed  so  as  to 
have  a  definite  ratio  between  their  cold  and  hot  resistance,  have 
been  calculated  in  the  same  way. 


Conductivity  of  flashed 

Cold  resistance. 

filament  (hot). 

a 

ft 

a 

Conductivity  of  filament 
of  equal  diameter  not 

Hot  resistance. 

flashed  (hot). 

1-6 

1-167 

1-43 

1715 

10-62 

1-7 

1-362 

1-43 

1469 

10-09 

14 

1-6 

1-43 

1250 

9-52 

1-9 

1-9 

1-43 

1053 

9-02 

2-0 

2-28 

1-43 

876 

8-46 

2-1 

2-83 

1-43 

707 

7-91 

2-2 

3-52 

1-43 

568 

7-35 

2-3 

4-62 

1-43 

433 

6-7^ 

2-4 

6-44 

1-43 

611 

6-01 

Curves  for  current  and  diameter  for  flashed  filaments  having 
these  ratios  of  cold  to  hot  resistance  might  be  plotted  on  Fig.  25 
along  with  the  "time  of  flashing"  curves  D,  E,  F,  &c.,  which 
they  would  cross.  To  save  confusion  one  only  (curve  K')  has 
been  drawn. 

In  many  lamp  factories,  the  sizes  of  filaments  and  the  amount 
of  flashing  they  receive  are  regulated  entirely  by  guess  and  trial 
methods.  Such  methods,  no  doubt,  answer  the  purpose  in 
the  end  just  as  well  as  a  calculation  by  a  formula,  or  informa- 
tion obtained  from  a  curve.  But  by  a  proper  understanding 
of  the  effects  produced  by  alterations  in  the  diameter  of 
the  filament  or  the  time  or  conditions  of  the  flashing,  the 
required  results  can  be  much  more  certainly  and  quickly 
obtained.  Of  course,  it  is  impossible  to  get  lamps  all  exactly 
right  by  any  method.  There  are  too  many  causes  at  work  to 
upset  the  calculations,  too  many  variable  conditions  to  be 
simultaneously  taken  into  account.  Nine  out  of  ten  of  the 
things  to  be  considered  may  be  right,  the  tenth  one  being 
wrong  throws  Ibhe  combined  result  wrong.  It  is,  however,  only 
by  taking  everything  into  consideration  that  good  results  can 
be  expected.  . 

The  curves  and  the  constants  given  for  use  with  the  various 
formulas  have  about  the  values  to  be  met  with  in  ordinary 
practice.  Carbons  produced  by  different  factories  using  very 
similar  processes  will,  however,  differ  so  much  that  it  is  not 


SIZES   OF   FILAMENTS    (FLASHED).  97 

possible  to  give  any  values  which  will  be  generally  applicable. 
The  exact  values  of  the  constants  must,  therefore,  be  found 
for  each  particular  make  of  carbon  and  process  of  flashing. 

A  good  instance  of  the  happy-go-lucky  methods  sometimes 
used  in  a  factory  was  seen  by  the  Author  a  few  years  ago. 
The  filaments  were  here  flashed  in  an  apparatus,  using  an 
automatic  cut-off,  which  stopped  the  current  when  it  attained 
a  certain  value.  This  cut-out  was  a  very  crude  affair,  and 
was  not  at  all  particular  as  to  when  it  went  off.  The  flashing 
globes  were  allowed  to  get  so  black  that  the  operator  could 
not  possibly  gauge  the  temperature  of  the  filament,  and  the 
consequence  was  that  the  cut-off  would  "act  "  just  when  the 
operator  chose  to  turn  the  current  up  sufficiently.  After  this 
process  the  filaments  were  "selected"  by  measuring  their 
resistance  cold,  a  range  of  about  twenty  per  cent,  being 
allowed.  The  result  was  that  three-fourths  of  the  filaments 
were  rejected.  Of  those  coming  within  the  limits  of  the  resist- 
ance cold,  perhaps  one-third  would  be  of  the  right  resistance 
hot,  which  was  always  assumed  to  be  half  the  cold  resistance. 
The  proportion  of  lamps  of  the  proper  voltage  was,  of  course, 
extremely  small,  while  the  percentage  of  filaments  which 
became  lamps  of  the  right  voltage  was  microscopical. 

The  factory  above  alluded  to  was,  luckily,  situated  in  a 
country  where  very  badly-matched  lamps  could  be  sold  as  of  the 
same  voltage.  It  will  be  evident  to  the  reader  that  the 
extreme  care  required  in  the  production  of  uniform  and  good 
lamps  necessarily  adds  to  their  cost.  Lamps  made  without 
such  care  can  be  produced  at  a  correspondingly  lower  rate. 
If  the  proper  amount  of  care  is  taken  during  the  manufacture, 
nearly  all  the  lamps,  when  finished,  will  be  close  enough  to 
the  proper  voltage  and  candle-power  for  them  to  be  properly 
sold  as  such.  When,  however,  such  care  is  not  maintained, 
the  resulting  lamps  will,  inevitably,  be  un-uniform,  only  a 
small  percentage,  possibly,  being  within  the  proper  limits  of 
voltage  and  candle-power. 

With  the  expiration  of  the  lamp  monopoly  in  this  country, 
it  is  to  be  feared  that  the  market  will  be  deluged  with  inferior 
and  badly  matched  lamps,  which  will  be  offered  at  a  very  low 
price.  Consumers  will,  however,  probably  find  that  the  higher 
priced  lamps  are  the  best  and,  in  the  long  run,  the  cheapest. 


CHAPTER  VIII. 


MEASURING  THE  FILAMENTS. 

THE  filaments  must,  of  course,  be  measured  before  mounting 
and  flashing,  and  it  is  well  to  measure  them  again  after 
flashing,  to  see  that  the  increase  in  thickness  is  of  the  proper 
amount.  The  measurement  may  be  done  in  several  ways — by 
means  of  micrometer  gauges,  or  by  throwing  a  magnified 
image  of  the  filament  upon  a  screen  by  means  of  a  lamp  and 
lens,  or  by  the  ordinary  microscope  method,  which,  however, 


32nds. 
I    .0312 
3   .0937 
'5  .1562 
7  .2187 
9 .2812 
11  .3437 
!3.4062 
15.4687 


1-8  .125 
t-4  2.  5  0 
18  .373 
ICths. 
1   .0625 
3    .1875 
5    .3125 
7  .437 


FIG.  26. — Micrometer  Gauge,  for  Measuring  Diameter  of  Filaments. 

cannot  be  considered  a  factory  method.  Measuring  with  a 
micrometer  can  only  be  done  with  accuracy  after  a  consider- 
able amount  of  practice.  The  best  form  of  micrometer  to  use 
is  that  represented  in  Fig.  26,  which  can  be  read  to  the  fifth 
part  of  a  mil.  It  should  be  mounted  on  a  support  fixed  to 
the  table,  so  as  to  be  about  Sin.  above  the  table,  and,  so 
that  the  screw  head  can  be  turned  easily  with  the  thumb  and 
fore-finger  of  the  right  hand,  with  the  fore-arm  resting  com- 
fortably on  the  table.  The  filament  will  be  held  very  lightly 


100         THE    INCANDESCENT   LAMP   AND   ITS    MANUFACTURE. 

between  the  thumb  and  fore-finger,  or  between  the  first  and 
second  fingers  of  the  left  hand.  Great  delicacy  of  touch  is 
required,  as  the  filament  may  be  squeezed  to  a  considerable 
extent,  and  the  measurement  will  then  be  wrong.  Some  micro- 
meters have  a  ratchet-head,  so  that  they  may  be  screwed  up 
until  the  object  to  be  measured  is  held  just  so  tightly  that 
the  ratchet  head  slides  round  without  closing  the  gauge  any 
further.  This  arrangement  is,  however,  not  sufficiently  deli- 
cate for  filaments.  They  may  be  squeezed  much  too  tightly 
before  the  ratchet  comes  into  play.  Of  course  a  ratchet -head 
might  easily  be  made  to  work  with  a  very  slight  pressure,  but 
the  difficulty  is  to  get  one  which  will  turn  the  micrometer 
screw  without  anything  in  the  jaws  (a  considerable  force  being 
required  to  do  this),  and  yet  which  will  slip  on  the  slightest 
extra  resistance.  A  micrometer  without  the  ratchet-head  may 
be  used,  and  it  can  be  screwed  up  until  it  is  felt  that  the  fila- 


M  IOOO"<S20   IS    10    5 

SWGI     4     8    12   16    20 .24.28.  32 


FIG.  27. — Trotter's  Micrometer  Gauge. 

ment  is  being  touched ;  but  it  is  difficult  always  to  squeeze  to 
the  same  extent.  A  much  more  accurate  plan  is  to  slowly 
move  the  filament  backwards  and  forwards  in  the  direction  of 
its  length  between  the  jaws  of  the  micrometer,  which  are 
steadily  and  slowly  being  screwed  up  with  the  other  hand. 
Immediately  the  filament  is  gripped,  no  matter  how  lightly,  it 
will  be  felt  by  the  fingers  holding  it — before  the  fingers  of  the 
other  hand  turning  the  micrometer  feel  the  resistance  at  all. 
Filaments  can  in  this  way  be  measured  very  accurately.  When 
the  filaments  are  not  properly  circular  there  is  sometimes  a 
difficulty  in  getting  the  greatest  measurement,  as  the  micro- 
meter tends  to  turn  the  filament  round  so  that  it  measures 
always  its  least  diameter. 

Another  instrument  which  can  be  used  in  the  same  way,  is 
the  "  Trotter"  wire-gauge,  the  screw  pattern  (Fig.  27).  It  is 
not,  however,  intended  for  measuring  so  fragile  a  thing  as  a 
filament,  and  is  not  so  convenient  as  the  ordinary  micrometer, 


MEASURING    THE    FILAMENTS. 


101 


as  it  indicates  on  a  vernier  which  is  so  small  as  to  require  a 
magnifying  glass  to  read  it.  It  has,  however,  this  advantage, 
that  the  jaws  come  together  without  the  turning  motion  of 
the  ordinary  micrometer. 

Fig.  28  represents  another  gauge  which  may  be  used.  It  is 
simply  a  metal  plate  with  a  V-shaped  slit  in  it,  into  which  the 
filament  can  be  passed  as  far  as  it  will  go.  The  sides  of  the 
V  are  marked  off  in  mils,  so  that  the  point  at  which  the  filament 
is  arrested  shows  its  thickness.  The  metal  plate,  preferably  of 


15 


10 


FIG.  28. — V-slot  Micrometer  Gauge. 

steel,  should  not  be  less  than  a  tenth  of  an  inch  in  thickness. 
Owing  to  the  angle  between  the  sides  of  the  V  being  so  very 
small,  there  is  a  danger  of  getting  the  filaments  down  a  little 
too  far,  and  getting  them  stuck  so  tightly  that  they  cannot  be 
got  out  without  breaking.  With  practice,  however,  this  instru- 
ment can  be  used  successfully. 

The  method  of  projecting  a  magnified  image  of  the  filament, 
or  a  part  of  it,  upon  a  screen  is  very  accurate  if  properly  carried 
out,  but,  if  the  measurements  are  to  be  made  as  quickly  as 
with  the  screw  micrometer,  it  requires  two  persons  to  work  it : 


102         THE    INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 

one  to  put  the  filaments  in  position,  and  the  other  to  read 
the  result  on  the  screen.     The   larger  the  image  the  more 
accurately  can  it  be  measured  on  the  screen.   There  are,  how- 
ever, two  difficulties  in  making  a  large  image.     Either  a  very 
short  focus  lens  must  be  used,  in  which  case  a  slight  error  in 
the  position  of  the  filament  will  make  a  considerable  error  in 
the  size  of  the  image,  or,  if  a  long  focus  lens  is  used,  the  dis- : 
tance  from  the  lens  to  the  screen  will  be  too  great.    In  any  case, 
an  achromatic  lens  should  be  used,  or  the  image  of  the  filament : 
will  be  blurred,  on  one  side  in  red  and  on  the  other  in  blue. 

If  the  filament  is  magnified  one  hundred  diameters,  the 
screen  may  be  divided  in  tenths  of  an  inch,  in  which  case  each 
division  will  represent  one  mil  in  the  filament.  The  screen 
may  be  made  to  slide  either  in  a  vertical  or  horizontal 
direction,  according  as  to  which  way  the  filament  is  held,  so 
that  the  zero  line  of  the  scale  may  be  brought  to  the  edge  of 
the  image  of  the  filament.  Another  method  is  to  use  a  scale 
drawn  on  glass,  the  image  of  which  is  thrown  together  with 
that  of  the  filament  on  to  the  screen.  There  is  a  slight  error 
here,  as  the  scale  must,  of  course,  be  either  in  front  or  behind 
the  filament,  and  will,  consequently,  be  either  a  little  too  large 
or  too  small.  This  error  will  be  unimportant  if  the  lens  be 
lin.  or  more  in  focal  length,  though  the  image  of  the  scale 
may  not  be  quite  sharp.  If  the  lens  has  a  focal  distance  of 
\lijic,  the  ^distance  of  the  screen  from  the  centre  of  the  lens 
will  be  8ft.,  5in.,  while  the  filament  will  be  1-Olin.  from 
the  cenf.ie  pf-the  lens.  This  is  rather  inconveniently  close  for 
tli6  handling  of  the  filaments,  but  a  greater  focal  length  of 
lens  requires  a  correspondingly  greater  distance  to  the  screen. 
A  lens  of  Sin.  focal  length  gives  more  room,  but  the 
distance  of  the  screen  will  then  be  over  25ft.  A  powerful 
illumination  is  required  in  order  that  the  image  on  the  screen 
may  be  easily  distinguishable.  An  incandescent  lamp  similar 
to  those  made  for  lantern  purposes  answers  very  well,  the  light  • 
being  concentrated  on  the  filament  by  a  condensing  lens  in 
the  usual  way.  This  method  is  very  useful  in  determining 
the  diameter  of  a  filament  after  it  has  been  made  into  a  lamp, 
the  filament  itself  being  rendered  incandescent.  The  filament 
is  placed  in  the  right  position  by  getting  it  in  line  between 
two  fixed  wires  or  narrow  slits  in  metal  plates  on  either  side. 


CHAPTER    IX. 


GLASS     MAKING. 

As  large  lamp  factories  frequently  make  their  own  glass,  a 
short  description  of  some  of  the  leading  features  of  the  manu- 
facture may  be  of  interest. 

The  glass  used  in  making  lamps  is  not  just  any  kind  of  glass, 
but,  on  the  contrary,  is  a  very  special  kind  of  good  glass.  This 
is  necessary,  owing  to  its  having  to  unite  properly  with  plati- 
num. The  glass  used  is  known  as  flint  glass.  It  contains  a 
large  quantity  of  lead,  and  is,  consequently,  much  heavier  than 
glass  without  lead.  All  the  materials  used  in  its  manufacture 
must  be  of  very  good  quality.  The  utmost  care  is  required  in 
making  up  the  mixture,  as  a  slight  deviation  from  the  proper 
proportions  of  the  ingredients  may  spoil  a  whole  potful  of 
glass. 

The  mixture  for  flint  glass  is,  approximately,  three  parts  by 
weight  of  pure  white  sand,  two  parts  of  red  lead,  and  one  part 
of  pearlash  (carbonate  of  potash),  with  small  proportions  of 
nitre,  arsenic,  and  oxide  of  manganese.  These  ingredients  are 
thoroughly  mixed  together  and  passed  through  a  very  fine  sieve. 

Experimenting  in  glass-making  is  a  very  expensive  amuse- 
ment, and  the  result  is  that  the  exact  recipe  for  making  any 
particular  glass  is  usually  difficult  to  obtain.  The  man  who 
knows  (the  " mixer"  as  he  is  called)  is  very  particular  to  do  his 
mixing  in  strict  privacy.  He  may,  perhaps,  have  a  boy  to  help 
him  weigh  out  the  three  main  ingredients,  but  no  one  may 
see  Him  do  the  rest.  The  Author  knew  a  mixer  who  went  so 
far  as  to  keep  a  ferocious  bull-dog  in  the  mixing-room,  pre- 
sumably to  prevent  any  one  from  interrogating  the  scales  in 
his  absence.  At  a  well-known  art  glass-works  it  is  said  that 
the  owner  alone  knows  the  mixtures  of  the  glasses,  and  makes 


104         THE   INCANDESCENT    LAMP   AND    ITS    MANUFACTURE. 

them  up  himself.  Let  the  lamp  maker,  therefore,  beware  of 
placing  himself  at  the  mercy  of  a  single  mixer  who  may  fall 
ill  or  be  otherwise  incapacitated,  and  thereby  stop  the  whole 
works.  Fortunately  the  secrets  of  mixtures  suitable  for  incan- 
descent lamp  glass  are  widely  known,  and  thus  the  only 
difficulty  is  to  find  a  mixer  who  can  be  relied  upon  to  be 
sufficiently  careful  in  doing  his  work. 

Every  batch  of  glass  should  be  carefully  tested  by  complet- 
ing a  dozen  lanips  or  so  from  it  before  the  rest  of  the  bulbs 
are  allowed  to  be  used.  The  glass  may  appear  to  be  all  right, 
but  after  the  lamps  have  been  made  a  few  days,  it  may  be 
found  to  crack  at  the  platinums.  All  the  bulbs  made  from 
each  batch  of  mixture  should,  therefore,  be  kept  apart  until  it 


FIG.  29.— Glass  Crucible  for  "  Cone  "  Furnace. 

is  known  that  the  glass  is  good.  Nothing  is  more  troublesome 
and  costly  than  to  get  bad  glass  into  the  factory  and  made 
into  lamps  before  the  fault  is  discovered. 

The  glass  furnace  is  usually  in  the  form  of  a  cone,  and  it 
generally  contains  six  or  eight  pots  or  crucibles  of  the  form 
shown  in  Fig.  29.  These  crucibles  are  made  of  fire-clay,  and 
are  entirely  closed  in  over  the  top.  The  opening  through 
which  the  glass  is  worked  is  at  the  side  near  the  top,  and  is 
surrounded  by  a  kind  of  hood  which  sticks  out  some  inches 
from  the  pot.  This  is  in  order  that  the  gases  from  the  fire 
may  not  come  into  contact  with  the  glass  in  the  pot  or  as  it 
is  being  withdrawn,  as  the  lead  would  then  get  reduced  and 
blacken  the  glass.  The  pots  are  arranged  round  the  circular 
bed  of  the  furnace  with  their  backs  inwards,  and  are  so  built 
in  that  their  mouths  only  are  visible  from  the  outside.  The 


GLASS    MAKING. 


105 


fire  is  in  the  centre  and  below  the  pots.  The  heated  gases  from 
the  fire  come  up  in  the  centre  and  strike  the  dome-shaped  crown 
of  the  furnace  and  are  deflected  towards  the  pots,  and  then 
pass  out  through  flues  between  the  pots  into  the  chimney  above 
the  crown.  There  are  various  devices  for  feeding  the  fire  from 
below  so  as  not  to  let  in  a  rush  of  cold  air  at  the  same  time. 
There  are  many  designs  of  furnace,  to  some  of  which  the 
Siemens  regenerative  system  is  applied.  For  fuller  descriptions 
of  furnaces,  works  on  glass  making  should  be  consulted. 

In  a  factory  where  the  large  output  of  a  cone  is  not  re- 
quired, a  smaller  furnace  may  be  constructed  to  accommodate 
three  or  four  small  pots  like  that  shown  in  Fig.  30.  These  pots, 
having  their  opening  in  the  top,  are  set  in  the  furnace  at  an 


FIG.  30.— Glass  Crucible  for  Small  Furnace. 

angle  of  about  thirty  degrees  with  the  horizontal,  and  the 
front  of  the  furnace  is  bricked  up  so  that  the  mouths  of  the  pots 
alone  are  seen  from  the  front.  The  firing  is  done  from  behind. 

Great  attention  has  to  be  paid  to  the  firing  of  a  glass  fur- 
nace in  order  to  keep  the  temperature  constantly  at  the  right 
point.  If  the  temperature  is  either  too  high  or  too  low,  the 
glass  cannot  be  worked.  Moreover,  a  variation  in  the  tempera- 
ture is  dangerous  to  the  pots,  which  will  crack  on  the  slightest 
provocation.  There  is  something  very  mysterious  about  the 
behaviour  of  the  crucibles  in  a  glass-furnace.  Sometimes  they 
will  last  for  months,  or  they  may  break  in  a  few  hours.  Large 
glass-works  make  their  own  crucibles,  but  they  can  be  bought 
ready-made.  Spare  pots  should  always  be  on  hand,  as  they  take 
a  long  time  to  make,  or  rather  a  long  time  must  elapse  after 


106         THE    INCANDESCENT   LAMP    AND   ITS    MANUFACTURE. 

making  before  they  can  be  used,  as  they  must  stand  to  dry  and 
harden.  A  glass  furnace  can  never  be  allowed  to  go  out,  un- 
less it  is  for  extensive  repairs.  All  the  pots  are  lost  if  the 
temperature  is  lowered  much.  When  a  pot  cracks  and  has  to 
be  replaced,  the  new  one  is  slowly  heated  in  a  separate  furnace, 
and  then  transferred  to  its  position  in  the  glass  furnace.  This 
is  a  somewhat  difficult  operation  owing  to  the  great  heat. 
Before  a  new  pot  is  charged  with  a  glass  mixture  it  must  be 
glazed  inside  with  glass  from  another  pot.  The  glass  mixture, 
together  with  scraps  of  glass,  can  then  be  poured  in.  It  will 
take  twenty-four  hours  or  more  before  the  glass  is  ready  to  be 
worked.  Besides  the  trouble  of  the  pots  being  liable  to  crack 
at  any  moment  without  notice,  there  is  another  danger  to 
which  they  are  exposed.  The  glass  mixture  will  sometimes 
act  upon  the  material  of  the  pot  and  make  holes  in  it,  which 
may  eventually  go  right  through  and  let  the  molten  glass  out 
into  the  furnace.  The  pots  are  sometimes  literally  honey- 
combed in  this  way,  though,  of  course,  as  soon  as  one  hole  is 
through,  the  pot  is  useless  for  further  work.  This  is  a  most 
troublesome  accident,  and  one  against  which  it  is  difficult  to 
guard  with  certainty. 

As  the  furnace  must  be  kept  up  continuously  night  and  day, 
it  is  best  to  work  the  glass  all  the  time  so  as  to  get  the  full 
money's  worth  out  of  the  fuel.  The  plan  in  some  glass-works 
is  to  charge  the  pots  on  Saturday ;  the  glass  will  then  be 
ready  for  working  on  Monday  morning,  from  which  time  it 
will  be  worked  continuously  night  and  day  until  the  following 
Saturday,  when  the  pots  will  be  again  filled  up. 

A  lamp  bulb  is  one  of  the  very  simplest  things  which  a  glass 
worker  can  have  to  make.  Everything  which  is  made  direct 
from  the  glass  pot  is,  in  fact,  made  first  of  all  into  a  bulb  in 
some  form  or  other.  Pressed  glass  is,  perhaps,  the  only 
exception,  as  it  is  merely  a  lump  of  glass  dropped  into  a 
mould  and  pressed.  This,  however,  does  not  mean  that  there 
is  no  skill  required  in  the  making  of  a  lamp  bulb.  The 
method  of  working  is  as  follows  :  The  glass  worker  has  an 
iron  tube  some  five  or  six  feet  long.  He  dips  the  end  of  it, 
after  first  heating  it, into  the  molten  glass  and  "gathers  "  some 
glass  on  the  end.  There  is  considerable  skill  required  in 
gathering  the  right  amount.  He  then  withdraws  the  tube, 


GLASS    MAKING. 


107 


and  holding  it  out  in  a  somewhat  inclined  position,  the  end* 
with  the  glass  upon  it  being  higher  than  the  near  end,  he 
blows  through  the  tube,  at  the  same  time  turning  it  round 
and  round,  and  in  this  way  forms  the  glass  into  a  hollow  bulb. 
If  he  wants  the  bulb  to  assume  a  long  or  pear  shape,  he  whirls 
the  tube  round  and  round  as  one  may  twirl  a  walking  cane. 
It  requires  much  skill  to  produce  bulbs  of  the  same  size  and" 
shape  in  this  manner,  but  good  glass  workers  are  able  to  make 
surprisingly  uniform  bulbs  in  this  way. 

There  is,  however,  another  method,  not  requiring  so  much 
skill,  by  which  the  bulbs  are  all  made  exactly  of  the  same  size. 
They  are  blown  into  a  mould,  such  as  is  shown  in  Fig.  31. 


FIG.  31.— Bulb  Mould. 

The  mould  is  made  of  iron,  and  is  lined  with  plumbago.  The 
glass  worker,  having  gathered  some  glass  on  his  iron  tube, 
stands  on  a  low  platform  or  stool,  and  places  the  end  of  the 
tube  with  the  glass  on  it  into  the  opening  of  the  mould,  which 
a  boy  holds  in  position  on  the  ground.  He  blows  down  the 
tube,  and  twirls  it  round  between  his  hands  at  the  same  time. 
The  glass  is  blown  out  so  that  it  fills  up  the  space  in  the 
mould.  The  boy  then  opens  the  mould,  and  releases  the 
bulb,  which  is  then  cut  off  from  the  tube.  Wooden  moulds 
may  be  used  if  only  a  few  bulbs  of  an  odd  size  are  wanted. 
Apple  wood  answers  the  purpose.  The  bulb  is  blown  in  just 
the  same  way.  The  surface  of  the  mould  quickly  becomes 
charred  or  carbonised,  and  thereby  protects  the  wood  below 
the  surface.  A  number  of  bulbs  may  be  made  in  it  before  it 
perceptibly  increases  in  size.  Pump  bulbs  and  the  like  may 


108         THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

very  well  be  blown  in  wooden  moulds,  as  also  may  the  globes 
or  shades  for  the  flashing  apparatus.  For  these  latter  the 
mould  need  not  be  so  complete.  It  is  sufficient  if  it  be  simply 
a  cylinder  open  at  both  ends. 

The  only  other  article  required  in  a  lamp  factory,  made 
directly  from  pot  glass,  is  glass  tubing.  Quantities  of  this  of 
various  sizes  are  required.  Small  sizes  are  used  for  making  the 
tubes  which  are  joined  to  the  bulbs,  and  through  which  the  air 
is  exhausted ;  and  larger  sizes  for  the  mercury  pumps  and  other 
apparatus.  The  method  of  making  the  tubing  is  very  simple. 
The  tube  begins,  as  most  articles  do,  with  a  bulb  at  the  end  of 
the  glass  worker's  iron  tube.  A  lump  of  glass  is  first  gathered 
from  the  pot  upon  the  end  of  an  iron  rod,  which  is  then  given 
to  a  boy  to  hold,  glass  uppermost.  The  glass  worker  then 
gathers  glass  on  his  iron  tube,  in  quantity  according  to  the 
size  of  the  tube  he  is  going  to  make,  and  forms  it  into  a  hollow 
bulb.  If  the  tube  is  to  be  of  large  diameter  it  will  be  a  large 
bulb,  and  will  require  a  good  deal  of  rolling  on  an  iron  plate, 
and  re-heating  at  the  mouth  of  the  crucible  before  it  is  of  the 
proper  shape.  It  must  be  perfectly  round  inside  and  out,  or 
the  tube  will  contain  along  its  whole  length  any  defect  in  the 
shape.  Having  got  the  bulb  into  the  required  size  and 
form,  the  glass  worker  turns  it  over  so  that  the  end  of  it  is 
brought  into  contact  with  the  glass  at  the  end  of  the  rod  which 
the  boy  is  holding  in  readiness  to  receive  it,  and  to  which  it 
will  adhere.  The  iron  rod  and  the  tube  are  then  both  held 
horizontally,  and  are  turned  round  and  round,  while  the  boy 
at  the  same  time  walks  backwards,  thus  elongating  the  bulb, 
and  drawing  it  out  into  a  long  tube.  The  tubing  is  made  in 
this  way  many  feet  long.  For  a  considerable  length  in  the 
centre  of  the  piece  it  is,  probably,  of  nearly  uniform  size, 
though  towards  each  end  it  will  be  larger.  When  it  is  pulled 
out  sufficiently,  and  becomes  rigid,  it  is  laid  on  the  floor  across 
pieces  of  wood  placed  ready  to  receive  it,  and  is  then  cut  up  into 
lengths,  which  are  afterwards  sorted  into  the  different  sizes. 
A  good  glass  worker  can  so  manipulate  the  glass  as  to  produce 
within  a  very  little  the  exact  size  of  tube  which  is  required. 

All  the  rest  of  the  work  on  the  glass  is  done  by  glass 
blowers  with  a  blow-pipe  flame,  and  will  be  treated  of  in  the 
next  chapter. 


CHAPTER    X. 


GLASS     BLOWING. 

THE  art  of  glass-blowing  consists  in  softening  glass  in  a, 
blow-pipe  flaine,  and  then  working  it  into  the  desired  shape  or 
form.  The  incandescent  lamp  industry  has  had  to  produce  its 
own  glass  blowers.  In  the  early  days  of  incandescent  lamp 
making  there  were  few  skilled  glass  blowers,  and  many  of  them 
were  only  skilled  in  making  perhaps  one  particular  article. 
Thus,  a  man  who  was  used  to  making  spiral-bulbed  thermo- 
meters would  in  all  probability  be  a  very  poor  hand  at 
"sealing  in"  a  lamp  filament.  He  would  demand  an  extor- 
tionate rate  of  pay,  and  would  work  very  slowly,  and  he 
would  not  work  at  all  if  anyone  was  standing  by,  as  he  would 
not  wish  to  give  away  the  secrets  of  his  trade.  It  was  on 
account  of  this  kind  of  thing  that  Messrs.  Wright  and  Mackie 
brought  out  their  glass-blowing  machine,  a  machine  specially 
designed  for  making  lamp-bulbs  out  of  tube,  and  for  sealing 
in  the  filaments,  all  without  the  aid  of  the  professional  glass 
blower.  Bulbs  made  straight  from  the  glass-pot — "  pot  bulbs," 
as  they  are  sometimes  called — were  not  made  at  the  time. 
The  bulbs  were  all  made  from  the  tube.  The  glass-blowing 
machines,  however,  never  came  into  general  use,  partly  on 
account  of  the  introduction  of  pot  bulbs  and  partly  because  it 
was  found  that  a  new  race  of  glass  blowers,  without  the  pre- 
judices of  the  old  hands,  was  springing  into  existence.  It  was 
found  that  lads  who  were  handy  with  their  fingers  could  soon 
learn  not  only  to  seal  in,  but  even  to  make  good  bulbs,  and 
many  of  them  even  to  do  the  more  difficult  work  of  making 
pumps.  A  lad  or  a  girl  at  a  fraction  of  the  wage  of  the  pro- 
fessed glass  blower  would  do  more  work,  and  yet  be  well  paid. 


110         THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

Complicated  glass  work  for  chemical  and  other  apparatus, 
which  many  glass  blowers  were  accustomed  to  make,  is  of 
necessity  done  in  a  very  slow  and  deliberate  manner.  A 
moment  of  impatience,  too  quick  heating  or  insufficient  an- 
nealing of  a  part,  might  spoil  a  whole  day's  work.  This  is 
no  doubt  the  reason  why  glass  blowers  appear  to  work  so  very 
slowly.  Such  a  slow  rate  is,  however,  quite  unnecessary  in 
lamp  making.  The  really  skilled  glass  blowers  are  not  found 
in  lamp  factories,  for  the  reason  that  they  are  not  required, 
and  that  it  pays  them  much  better  to  make  chemical  and  other 
apparatus,  which  always  commands  a  good  wage. 

The  blow-pipe  most  commonly  used  in  glass-blowing  is  that 
called  the  "cannon."  Various  forms  of  this  blow-pipe,  of  a 


11  1 1 

FIG.  32.— Cannon  Blow-Pipe. 

more  or  less  elaborate  construction,  are  made,  but  the  simplest 
and  cheapest  form,  shown  in  Fig.  32,  is  the  most  satisfactory. 
It  consists  of  a  piece  of  brass  tube,  A,  open  at  both  ends, 
joined  to  another  tube,  B,  at  about  the  angle  shown  in  the 
figure.  C  is  a  screw  stop-cock.  D  is  a  socket  through  which 
the  tube  B  slides,  so  that  the  whole  can  be  raised  or  lowered 
and  fixed  by  the  thumb-screw  in  D  at  any  convenient  height 
above  the  table  E.  Below  the  table  the  tube  B  is  joined  by  a 
flexible  tube  to  the  gas  supply.  F  is  a  cork  with  a  hole  bored 
through  its  centre  to  accommodate  a  piece  of  glass  tube,  G. 
About  two-thirds  up  the  tube,  A,  three  screws  pass  through  it 
and  act  as  guides  to  the  inner  glass  tube,  G,  and  keep  it  in 
the  centre.  The  end  of  the  glass  tube  G  is  joined  by  a  piece 
of  flexible  tube,  H,  to  the  air-blast  through  the  stop-cock  J. 


GLASS    BLOWING, 


111 


Almost  any  kind  and  size  of  flame  may  be  obtained  with 
this  blow-pipe,  depending  on  the  regulation  of  the  gas  and  air 
blast  by  the  cocks  C  and  J.  It  is  necessary,  however,  to  use 
a  larger  bore  glass  tube,  G,  for  the  large  flame  than  for  the 
smaller  ones,  while  for  the  small  pointed  silent  flame  the 
nozzle  of  G  must  be  considerably  contracted.  It  is  a-  matter 
of  importance  that  this  glass  tube  be  quite  straight  and  regular 
and  smooth  inside,  and  that  both  ends  of  it,  especially  that 


FIG.  33.— Form  of  Large  Flame  of  Cannon  Blow-Pipe. 

from  which  the  air  issues,  be  cut  off  perfectly  straight  across 
and  at  right  angles  to  the  tube.  This  tube  can  be  moved  in 
and  out  through  the  cork  and  adjusted  to  its  proper  position. 
It  should  not  reach  to  the  end  of  A  by  a  quarter  of  an  inch 
or  so. 

In  working  with  French  or  German  glass,  which  does  not 
contain  lead,  no  great  attention  need  be  paid  to  the  flame,  and 


FIG.  34.— Pointed  Blow-Pipe  Flame. 

any  part  of  it  may  be  used.  As,  however,  lead  glass  is  used 
almost  always  in  lamp -making,  it  is  necessary  to  take  care 
that  the  flame  is  a  suitable  one,  called  sometimes  a  "clean  " 
flame.  That  is  to  say,  it  must  be  one  in  which  the  "reduc- 
ing "  and  the  "  oxidising  "  parts  are  clearly  denned  and  are 
not  mixed  up.  Figs.  33  and  34  show  the  forms  of  the  cannon 
and  the  pointed  flames  respectively.  The  "  reducing  "  flame 
is  that  part  marked  B.  It  contains  an  excess  of  carbon,  while 
in  the  "oxidising"  flame,  C,  there  is  an  excess  of  oxygen. 


112          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

These  different  parts  of  the  flame  can  be  easily  distinguished 
by  their  appearances.  The  hottest  part  of  the  flame  is  just  at 
the  junction  of  B  and  C,  at  the  tip  of  the  point  of  B  in 
Fig.  34. 

In  working  with  lead  glass,  the  oxidising  part  C  can  alone 
be  used.  If  the  glass  is  held  in  B,  some  of  the  lead  is  imme- 
diately reduced  to  the  metallic  state,  and  colours  the  glass 
black.  When  this  takes  place  it  is  at  once  seen  by  the  lead 
appearing  as  a  bright  red  opaque  patch  on  the  otherwise 
nearly  transparent  glass.  If  allowed  to  cool,  the  patch  will  be 
found  to  be  black.  When  this  patch  of  lead  appears,  it  can 
easily  be  oxidised  again  by  holding  it  further  away  in  the  G 
part  of  the  flame.  This,  however,  can  only  be  done  if  the  patch 
is  superficial.  If  the  lead  has  been  reduced  much  below  the 
surface,  it  is  impossible  to  get  the  glass  clear  again.  The 
reason  for  being  so  particular  about  the  inner  or  wind  tube  of 
the  blow-pipe  is  that  a  tube  which  is  irregular  in  any  way 
will  impart  its  unevenness  to  the  air  blast,  so  that  the  oxidis- 
ing and  reducing  parts  of  the  flame  are  all  mixed  up,  or  rather 
there  is  no  oxidising  part.  Blow-pipes  with  metal  wind  tubes, 
which  can  be  purchased  ready  made,  are  generally  unsuitable 
for  lead-glass  working  on  this  account. 

The  blow-pipe  is  fixed  on  the  table  about  four  or  five  inches 
from  the  edge,  and  pointing  directly  away  from  the  operator. 
The  table  should  be  covered  with  a  thick  sheet  of  asbestos, 
and  this  should  be  painted  black,  as  the  flame  can  be  seen 
much  better  against  a  dark  background.  The  glass-blowing 
table  should  not  be  brilliantly  lighted,  and  the  light  should 
come  from  above,  or  from  the  sides,  and  not  from  in  front  of 
the  operator.  If  there  is  too  much  light,  it  is  difficult  to  see 
the  flame,  or,  at  any  rate,  the  oxidising  part  of  it,  which  is 
almost  entirely  non-luminous. 

There  is  one  objection  to  the  cannon  form  of  blow-pipe 
for  large  flames.  It  is  very  noisy.  In  a  room  with  a  number 
of  them  at  work  it  is  a  matter  of  some  difficulty  to  hold  any 
conversation,  on  account  of  the  powerful  roar  which  they 
make.  The  small  pointed  flame  (Fig.  34),  however,  makes 
no  noise  whatever,  and  for  this  reason  it  is  better  to  use  a 
compound  blow-pipe  made  up  of  several  small  noiseless 
flames.  This  may  be  accomplished  by  fixing  three  or  four 


GLASS   BLOWING. 


113 


blow-pipes  giving  such  flames  side  by  side,  and  so  directed 
that  the  tips  of  the  flames  meet  each  other.  To  obtain  a 
greater  heating  power  an  equal  number  may  be  arranged 
opposite  to  the  first  set,  in  the  reverse  direction,  so  that  all 
the  tips  of  the  flames  meet  in  the  centre  of  the  space  between 
the  two  sets  of  blow-pipes.  In  this  way  is  produced  an 
excellent  and  perfectly  quiet  blow-pipe  flame,  which  gives  a 
more  even  heating,  the  glass  being  heated  on  both  sides  at 
the  same  time,  though  this  is  not  always  an  advantage.  Such 
an  arrangement,  however,  is  only  applicable  to  work  which 
does  not  require  a  varying  flame.  It  is  suitable  for  sealing  in 


FIG.  35. — Compound  Blow-Pipe. 

filaments,  though  the  pump-maker  could  not  work  with  it 
entirely.  It  is  certainly  an  advantage  to  have  the  sealing-in 
room  quiet.  Six  or  eight  cannon  blow-pipes  set  up  together 
in  the  way  just  described  is,  however,  a  rather  clumsy  arrange- 
ment. A  better  plan  is  to  use  blow-pipes  of  the  form  shown 
in  Fig.  35.  A  is  the  gas  tube,  B  is  the  air  tube,  each  of  which 
is  provided  with  a  stop-cock.  At  the  top  A  terminates  in  a 
hollow  burner,  through  which  the  gas  passes  by  a  number  of 
small  holes.  The  air  tube  B  terminates  in  a  glass  nozzle  C. 
The  gas  issuing  from  the  top  of  A  having  been  lighted,  the 
stop-cock  in  B  is  turned,  with  the  result  that  the  air  coming 
from  C  blows  into  the  luminous  gas  flame,  and  makes  it 

i 


114         THE   INCANDESCENT   LAMP   AND   ITS   MANUFACTURE, 

assume  the  pointed  quiet  type  of  blow-pipe  flame.  Six  or 
eight  of  these  blow-pipes  may  be  arranged  so  that  the  flames 
are  combined  together,  as  shown  in  plan  in  Fig.  36.  The  six 
pointed  flames  burn  no  more  gas  than  the  single  cannon 
flame  suitable  for  the  same  work  would  do  ;  probably  they  burn 
less.  The  compound  form  is,  of  course,  more  costly  in  the 
first  instance.  There  are  stop-cocks,  controlling  both  the  air 
and  the  gas  of  the  whole  six  burners,  so  that,  in  case  of  any 
increase  or  decrease  in  the  pressure  of  either,  it  is  not 
necessary  to  alter  the  whole  six  or  twelve  cocks  ;  in  fact,  when 
the  stop-cocks  in  the  pillars  are  once  set  properly  they  should 
not  be  again  disturbed. 

It  is  essential  that  the  glass-blowing  room  be  free  from 
draughts,  as  a  slight  draught  will  blow  the  flame  to  one  side, 
and  render  working  difficult.  As  a  consequence  of  this,  the 


FIG.  36. — Form  of  Flame  of  Compound  Blow-Pipe. 


state  of  the  atmosphere  in  a  glass-blowing  room  with  a  number 
of  blow-pipes  working  is  often  distressing  to  anyone  but  a 
glass-blower.  There  is  no  reason  why  a  glass-blowing  room 
should  not  be  lofty  and  well  ventilated,  without  any  incon- 
venient draughts,  but  this  is  seldom  the  case.  The  Author, 
not  long  ago,  had  an  opportunity  of  going  over  a  new  lamp 
factory,  the  building  having  been  constructed  for  the  purpose, 
and  was  surprised  to  find  that  even  under  such  circumstances 
the  glass-blowing  room  had  a  very  low  ceiling,  and  no  means 
of  ventilation  whatever.  The  quantity  of  gas  used  by  a 
blow-pipe  is  very  considerable,  and  the  ventilation  should,  of 
course,  be  ample  and  easily  controlled. 

Each  glass-blower  can,  by  means  of  a  foot  bellows,  pump 
the  wind   for  his   own  blow-pipe.      This    method   has    an 


GLASS   BLOWING.  115 

occasional  advantage  in  enabling  him  to  vary  the  pressure, 
and  therefore  the  form  and  heat  of  the  flame,  without  having 
to  turn  a  cock.  It  seldom  occurs,  however,  that  one  hand 
•cannot  be  spared  for  an  instant  for  this  purpose. 

The  disadvantage  of  a  foot  bellows  is  that  the  glass  blower 
•cannot  sit  so  comfortably  or  so  steadily,  both  important 
•considerations  in  glass  blowing,  when  he  has  to  keep  one  foot 
pumping  up  and  down  all  the  while.  Moreover,  the  wind  is 
apt  to  run  out  at  a  critical  moment.  If  a  foot  bellows  is  used 
it  is  best  to  use  a  much  larger  one  than  the  ordinary  labora- 
tory form,  and  to  have  it  fixed  underneath  the  table  with  a 
large  air  reservoir,  in  order  that  the  blast  may  be  steady,  and 
not  go  up  and  down  with  each  stroke.  The  bellows  and 
reservoir  should  be  large  enough  that  a  single  stroke  will 
supply  the  blow-pipe  for  some  minutes.  Such  an  arrangement, 
however,  does  away  to  a  great  extent  with  the  advantage 
mentioned  of  being  able  to  vary  the  force  of  the  blast  with 
-the  foot  at  any  moment,  which  can  only  be  done  with  a 
tellows  of  small  capacity.  In  a  factory  where  a  large  number 
•of  blow-pipes  are  required  it  is  necessary  that  a  supply  of 
air  be  laid  on  so  that  the  glass  blowers  do  not  require  separate 
bellows.  For  this  purpose  a  mechanical  pump  with  a  single 
•cylinder  driven  by  power  answers  very  well.  The  air  is  forced 
into  an  iron  tank  with  a  blow-off  valve.  The  pressure  may 
very  well  be  five  or  six  pounds  to  the  square  inch,  and  the 
blast  will  be  perfectly  steady  if  the  reservoir  is  large  enough. 
'The  force  of  the  blast  is,  of  course,  entirely  under  control  by 
the  screw  cocks  on  the  blow-pipes.  With  an  ordinary  foot 
bellows  the  pressure  is  only  a  small  fraction  of  this  amount, 
tut  a  much  larger  pressure  in  the  pipes  works  perfectly  well 
so  long  as  the  rate  at  which  the  air  is  admitted  into  the  flame 
is  under  control.  The  air  should  be  taken  from  the  pump 
into  the  side  of  the  reservoir,  the  bottom  of  which  should  be 
of  such  a  form  that  any  water  inside  may  drain  towards  one 
part  where  a  cock  can  be  placed  by  which  it  can  be  let  off. 
The  delivery  pipe  should  be  led  off  from  the  top,  so  as  to  be 
as  dry  as  possible.  In  damp  weather  there  is  always  water 
condensed  in  the  reservoir,  and  if  it  gets  into  the  pipes  it 
may  cause  trouble  at  the  blow-pipes.  Instead  of  a  pump,  a 
fan  may  be  used.  Fans,  however,  as  usually  constructed,  are 

i2 


116          THE    INCANDESCENT   LAMP   AND   ITS    MANUFACTURE. 

adapted  for  delivering  large  quantities  of  air  at  a  small 
pressure,  and  are,  consequently,  unsuited  for  supplying  air 
through  long  lengths  of  small  piping.  As  a  rule,  they  require 
a  very  high  speed,  and  are  very  noisy. 

As  lamp  bulbs  blown  from  glass  tubing  have  not  altogether 
gone  out  of  fashion  the  method  of  making  them  will  be 
described.  The  cannon  form  of  blow-pipe  with  the  noisy 
flame  is  the  best  to  use  for  this  purpose.  The  size  of  the 
glass  tube  from  which  the  bulbs  are  to  be  blown  will  be 
selected  according  to  the  size  of  the  bulb  which  is  required* 
For  an  ordinary  16  candle-power  size  it  will  be  about  fin. 
in  diameter,  and  about  -^th  of  an  inch,  or  rather  more, 
in  thickness.  The  tube  must  be  quite  clean  inside  and  out, 
as  any  dirt  may  get  so  burnt  into  the  glass  that  it  cannot 
be  removed.  The  glass  tubes  are  usually  cut  into  lengths 
of  about  3ft.  The  glass-blower  takes  one  of  these,  andr 
sitting  himself  at  the  table  with  the  blow-pipe  just  in  front 
of  him,  adjusts  the  size  of  the  flame,  using  more  gas 
at  first  than  he  requires  for  softening  the  glass,  making* 
thereby  a  flame  much  larger  and  not  so  hot  as  he  will 
ultimately  require.  He  then  holds  the  glass  tube  with  his  left 
hand,  so  that  one  end  of  it  is  in  the  flame  at  some  distance 
from  the  blow-pipe,  at  the  same  time  turning  the  tube  round 
and  round  so  that  it  shall  get  heated  evenly.  The  heating 
must  be  started  gradually  in  this  way,  or  the  tube  may  crack. 
If  the  tube  be  a  thick  one  it  will  be  necessary  to  turn  it  round 
and  round  at  a  point  beyond  the  flame  altogether,  so  as  to  get 
it  very  gradually  heated  before  it  is  brought  into  the  flame  at 
all.  As  the  tube  becomes  heated  the  gas  is  turned  down, 
thereby  reducing  the  size  of  the  flame,  and  making  it  hotter. 
The  end  of  the  tube,  constantly  being  rotated,  is  brought  into 
the  oxidising  part  of  the  flame,  and  soon  begins  to  soften. 
When  this  occurs  the  glass-blower  presses  the  ends  of  the  tube 
inwards  with  the  tail  end  of  a  file  or  other  convenient  instru- 
ment, so  that  the  sides  touch  one  another.  He  then  melts  on 
a  short  length  of  small  ''cane"  or  "  rod  "  glass.  Having 
done  this,  he  will  heat  the  tube  a  little  lower  down,  and  when 
it  is  soft  he  will  take  it  out  of  the  flame,  and,  by  means  of  the 
piece  of  cane  glass,  will  draw  out  the  heated  part  of  the  tube 
for  Sin.  or  lOin.  or  so,  as  shown  at  B  in  Fig.  37.  The  part 


GLASS    BLOWING. 


117 


drawn  out  will  be  narrow  and  thin.  When  the  tube  on 
which  the  glass-blower  is  working  is  long  he  cannot  conve- 
niently hold  it  up  and  work  it  at  the  same  time.  A  rest  must 
therefore  be  provided  to  hold  one  end  of  it.  If  the  end  is 
allowed  to  lie  on  the  table  it  .will  roll  along  as  the  tube  is 
rotated.  The  rest  must  therefore  carry  a  groove  or  notch  in 
which  the  tube  can  turn  without  rolling  along.  Having  drawn 
out  the  end  of  the  tube,  the  piece  of  cane  used  for  the  purpose 
is  no  longer  required,  and  may  be  detached.  The  tube  is 
then  heated  about  3in.  lower  down  and  drawn  out  as  before 
for  15in.  or  so  (C,  Fig.  37).  This  is  done  by  means  of 
the  first  piece  drawn  out,  which,  being  thin,  quickly  cools 
enough  to  allow  of  handling.  The  tube  is  then  cut  at  the 


FIG.  37.— Different  Stages  in  Blowing  a  Bulb  from  Glass  Tube. 

centre  of  the  narrow  part  of  the  second  drawing  out,  leaving 
a  piece  of  the  original  tube  about  Sin.  long  drawn  out 
at  each  end  into  a  small  tube,  D.  The  glass  is  cut  by 
first  scratching  it  with  a  file  and  then  pulling  it,  when  it 
will  part  at  the  scratch.  The  process  of  drawing  out  and 
cutting  off  is  then  repeated  until  the  whole  of  the  tube  has 
been  converted  into  pieces  similar  to  D.  Each  of  the  pieces 
*vill  make  a  bulb.  The  next  operation  is  to  close  up  one  end 
by  heating  in  the  flame.  This  is  easily  accomplished,  as  the 
glass  will  close  up  of  itself  as  soon  as  it  becomes  soft.  When 
tliis  is  done,  the  narrow  tube,  which  has  not  been  closed,  is 
heated  close  to  its  junction  with  the  full-sized  tube,  so  as 
to  make  a  contraction  in  the  bore,  E.  This  is  the  point 


118         THE    INCANDESCENT   LAMP   AND    ITS   MANUFACTC7KE. 

where  the  lamp  will  be  eventually  sealed  up  after  pumping. 
Nothing  remains  now  but  to  form  the  bulb.  For  this  pur- 
pose a  rather  larger  flame  is  required,  and  is  made  to  play 
upon  the  thick  part  of  the  tube,  E,  rather  nearer  to  the 
contracted  end  than  the  other.  As  the  glass  softens  it  caves 
in  all  round,  and  the  glass-blower  pushes  the  smaller  tubes- 
inwards,  so  as  to  get  the  soft  glass  into  a  smaller  space,  and  to 
get  it  better  covered  by  the  flame.  When  it  has  attained  the 
exact  degree  of  softness,  which  is  ascertained  as  much  by  the 
feel  as  the  appearance,  he  takes  it  out  of  the  flame,  and  holding 
it  with  the  open  end  to  his  mouth  in  a  horizontal  position 
and  quickly  rotating  it  all  the  while,  he  blows  into  it  with  a 
series  of  short  quick  puffs,  thereby  blowing  out  the  softened 
glass  to  any  required  extent,  and  forming  the  bulb  F.  The  bulb 
can  be  made  of  any  shape  according  to  the  way  the  soft  glass 
is  manipulated  and  blown  into.  Success  in  blowing  a  bulb 
depends  on  the  even  heating  of  the  glass,  and  skill  in  manipu- 
lating it  while  it  is  being  blown  into.  It  takes  a  good  deal  of 
practice  before  a  good  symmetrical  bulb  can  be  made  with 
certainty.  Beginners  invariably  make  one  side  larger  than 
the  other.  If  a  large  bulb  is  to  be  made  from  tubing,  a  quan- 
tity of  glass  must  be  run  together,  so  that  it  is  very  thick  at 
the  place  where  the  bulb  is  to  be  blown.  This  is  accomplished 
by  pressing  the  tube  lengthways  towards  the  softened  part,, 
thereby  bringing  in  more  and  more  glass  into  the  flame.  The 
exhausting  tube  of  the  bulb  F  should  be  closed  up  as  soon  as 
the  bulb  is  finished,  to  prevent  dust  from  getting  inside.  Bulbs 
blown  from  tubing  as  described  have  this  exhausting  tube 
already  attached.  Pot  bulbs,  however,  have  no  such  tube, 
and  one  must  consequently  be  joined  on.  This  has  to  be  done 
before  the  filament  is  put  in,  as  the  tube  is  necessary  for 
holding  the  lamp  during  that  operation.  Tubing  of  the 
required  size,  about  -Jin.  diameter,  is  cut  up  into  lengths 
of  about  Gin.  An  indiarubber  cap  is  slipped  over  the 
neck  of  the  bulb  to  make  it  air-tight.  The  end  of  the  bulb 
where  the  exhausting  tube  is  to  be  attached  is  then  held  in 
the  blow-pipe  flame,  being  rotated  as  usual,  so  as  to  soften  a 
small  part  of  the  glass  just  in  the  centre.  By  the  time  this 
spot  is  soft,  the  air  confined  within  the  bulb  has  become 
heated,  and  therefore  exerts  a  pressure  outwards,  which  is 


GLASS   BLOWING.  119 

sufficient  to  blow  the  softened  part  outwards  and  make  a  hole 
right  through.  The  bulb  is  usually  held  in  the  flame  and 
rotated  by  the  left  hand,  while  at  the  same  time  the  right 
hand  is  heating  the  tip  of  one  of  the  Gin.  glass  tubes.  As 
soon  as  the  hole  appears  in  the  top  of  the  bulb  the  softened 
end  of  the  tube  is  stuck  over  it,  and  the  junction  is  held  in 
the  flame  and  thoroughly  worked  and  melted  together,  and  to 
assist  the  operation  it  is  several  times  blown  into  and  swelled 
out  and  reduced  down  again,  finally  being  left  contracted  close 
to  the  bulb,  as  in  the  case  of  the  tube-blown  bulbs.  If  the 
bulb  is  not  required  for  use  for  some  time  the  ends  should  be 
sealed  up. 


, 


CHAPTER   XL 


SEALING-IN. 

"SEALING-IN  "the  filament  is  the  next  process  to  be  described. 
The  precise  details  of  the  process  depend  upon  the  way  the 
filament  is  mounted,  or  whether  the  platinum  wires  are  joined 
together  by  glass  in  any  way,  and  whether  the  lamp  is  to  be 
"  capped,"  or  is  to  have  the  platinum  wires  formed  into  loops 
at  the  bottom  or  sides  of  the  neck  of  the  bulb.  First  of  all, 
the  exhausting  tube,  if  closed  up,  must  be  opened  by  cutting 
off  the  tip.  This  tube  is  now  used  for  holding  the  bulb,  and 
it  makes  a  very  convenient  handle  by  which  to  rotate  the  bulb 
in  the  flame.  The  neck  of  the  bulb  is  then  heated  by  rotating 
it  in  the  blow-pipe  flame,  so  as  to  soften  the  glass  at  a  short 
distance  from  the  bulb.  When  soft  the  superfluous  piece  of 
neck  is  pulled  away  with  the  right  hand,  while  the  left  still 
holds  the  bulb  in  the  flame,  so  that  the  piece  of  neck  is  severed 
from  the  rest,  leaving  the  bulb  with  the  short  remaining  neck 
closed  up.  If  too  long  a  neck  still  remains,  more  maybe 
removed  by  again  heating  and  drawing  it  off  by  means  of  a 
small  piece  of  glass  rod  or  a  bit  of  waste  glass  from  a  former 
operation.  The  end  of  the  short  neck  has  now  to  be  opened - 
for  the  reception  of  the  filament.  It  is  again  held  in  the  flame, 
and  when  soft  the  small  tube  is  blown  through  somewhat 
violently,  so  that  a  hole  is  blown  or  a  large  and  very  thin  bulb- 
is  formed,  which  can  easily  be  removed  with  the  end  of  a  file 
or  other  instrument  (A,  Fig.  38).  The  opening  produced  is 
probably  too  small  to  get  the  filament  through,  and  it  must 
therefore  be  enlarged.  This  is  done  by  pressing  out  the  sides - 
when  soft  with  the  end  of  a  file,  as  at  B.  One  of  the  glass- 
blower's  most  useful  tools  is  a  small  fine  file,  as  with  it  he 


122          THE   INCANDESCENT   LAMP   AND    ITS    MANUFACTURE. 

can  scratch  the  glass  in  order  to  cut  it  at  any  point,  while 
the  end  which  is  intended  to  go  in  a  handle  can  be  used  for 
moulding  the  glass  when  soft.  The  filament  already  mounted 
on  the  platinum  wires  is  next  carefully  inserted  through  the 
opening.  It  may  be  handled  by  the  wires  by  means  of  a  small 
pair  of  pliers.  When  far  enough  in  the  bulb  the  wires  are 
held  against  the  glass  at  the  end  of  the  opening  and  held  in 
the  flame.  As  soon  as  the  glass  at  the  point  of  contact  is  soft, 
the  wires  will  adhere  to  it,  and  the  pliers  can  be  laid  down. 
The  next  thing  is  to  close  up  the  opening.  It  will  close  up  of 
itself  if  sufficiently  heated,  but  it  may  be  assisted  by  pinching 
with  a  pair  of  spring  nippers  made  for  the  purpose.  If  this 
is  done  there  will  be  rather  large  corners  formed,  as  in  C, 


FIG.  38.— Different  Stages  in  the  Process  of  "  Sealing-in  "  the  Filaments. 

which  can  be  cut  off  while  soft  with  a  pair  of  cutting  nippers. 
To  make  the  seal  good  and  neat  it  will  require  several 
times  heating  and  blowing  out  by  blowing  through  the  small 
tube. 

If  the  lamp  is  to  be  "  capped,"  the  end  must  be  formed  so 
that  the  plaster  used  in  the  cap  will  have  a  good  hold  on  the 
lamp.  It  must  be  shaped  so  that  it  can  neither  be  pulled  out 
of  the  plaster  nor  twisted  round  in  it.  The  simplest  way  of 
insuring  this  is  to  make  a  flat  piece  in  the  way  just  described 
with  a  thickened  end,  as  in  D  and  E.  The  flat  end  prevents 
the  lamp  from  turning  round,  and  the  thickened  part  is 
surrounded  by  the  plaster  and  cannot  be  pulled  out.  The 
thickened  end  may  be  made  by  melting  a  piece  of  glass  rod, 


SEALING  IX.  123 

and  winding  a  string  of  the  semi-fluid  glass  round  the  end  of 
the  flat  piece.  Or,  if  the  glass  in  the  neck  of  the  bulb  be 
thick  enough,  it  may  be  simply  squeezed  upon  the  wires,  but 
squeezed  thinner  close  to  the  bulb  than  at  the  end  where  the 
wires  come  through. 

Some  glass-blowers  adopt  a  somewhat  different  method  of 
sealing  in.  The  filament  is  inserted  in  the  opening  in  the 
bulb,  as  before,  but  is  allowed  to  slip  inside,  wires  and  alL 
The  neck  is  then  heated,  and  drawn  down  quite  small,  and 
sealed  up.  A  small  hole  is  now  made  in  the  end  by  blowing 
through  as  before.  The  bulb  is  tipped  up  so  that  the  wires 
protrude  by  the  required  amount.  The  small  opening  is  then 
easily  closed  up  as  before.  This  is  rather  a  neater  method 
than  the  other,  but  is  more  likely  to  injure  the  filament.  If 
the  wires  are  to  be  formed  into  loops,  they  may  be  bent  into 
shape  before  the  filament  is  put  into  the  bulb,  or  the  loops 
may  be  formed  after  the  seal  is  made.  The  first  method  pro- 
duces the  most  uniform  loops.  Care  must  be  taken  in  forming 
the  loops  that  they  are  of  equal  size,  or  there  may  be  a 
difficulty  in  making  contact  with  the  holder  to  which  the 
lamp  will  eventually  be  attached.  If  the  wires  are  to  be 
brought  out  at  the  sides  of  the  neck,  a  similar  method  may  be 
used,  the  filament  being  inserted  as  before,  and  the  ends  of 
the  wires  pushed  back  through  small  holes  made  in  the  neck. 
Another  method  is  to  cut  the  neck  off  at  the  place  where  the 
wires  are  required  to  come  out,  and  then  to  put  in  the  filament 
with  the  wires  reaching  over  the  sides  and  melt  the  neck  on 
again. 

In  an  ordinary  flint  glass  works  an  important  piece  of 
apparatus  is  the  annealing  oven.  Glass  which  has  been 
worked  into  various  shapes  and  joined  to  other  glass  requires 
to  be  very  slowly  cooled  down.  It  is  put  into  the  annealing 
oven,  and  given,  perhaps,  several  days  to  gradually  cool  to  the 
temperature  of  the  air.  If  allowed  to  cool  rapidly  it  will  pro- 
bably crack  either  before  it  is  quite  cold  or  after  an  interval 
of,  perhaps,  days  or  weeks.  In  lamp  factories  no  such  anneal- 
ing ovens  are  necessary.  If  really  good  glass  is  used,  and  the 
seal  is  properly  welded,  no  annealing  whatever  is  required. 
The  seal  may  be  allowed  to  cool  as  quickly  as  it  likes.  It  is, 
however,  customary  to  anneal  to  a  slight  extent.  It  may  be 


124 


THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


done  by  using  a  kind  of  oven  made  of  sheet  iron,  the  whole 
thing  being  quite  small,  each  glass-blower  having  one  on  his 
bench.  The  top  is  capable  of  being  rotated,  and  has  four 
circular  holes  in  it.  Below  three  of  the  holes  is  a  Bunsen  gas 
burner  with  a  rose  top.  One  is  turned  up  fully,  the  next  has 
a  smaller  flame,  and  the  third  a  still  smaller  one.  When  a 
lamp  is  sealed  in,  it  is  placed,  seal  downwards,  in  the  hole  over 
the  largest  flame.  The  hole  is  of  such  a  size  that  the  lamp 
rests  about  one-third  through  it.  The  seal  is,  therefore,  pre- 
vented from  getting  cold  at  once.  When  the  next  lamp  is 
sealed  in,  the  top  of  the  oven  is  turned  round,  so  that  the  first 
lamp  is  brought  over  the  second  gas  burner,  which  will  allow  it 


FIG.  39,-— Lamp  with  Annealing  Cap. 

to  cool  a  little  more,  and  the  new  lamp  is  placed  in  the  next 
hole,  which  is  now  over  the  first  burner.  The  next  lamp  is 
treated  in  the  same  way,  and  put  over  the  first  burner,  while 
the  others  are  turned  round,  so  as  to  be  over  the  second  and 
third  respectively.  On  a  fourth  lamp  being  sealed  in,  the  first 
one  will  be  over  no  burner  at  all,  and  will  get  quite  cold,  and 
when  a  fifth  lamp  is  ready  the  first  is  taken  out  of  the  annealer 
and  put  away.  In  this  way  a  lamp  is  made  to  cool  gradually 
in  about  a  quarter  of  an  hour,  and  this  is  quite  a  long  enough 
time  if  the  glass  is  good  and  the  seal  well  made.  Another 
method  is  to  put  a  glass  cap  or  cover  over  the  seal  (Fig.  39). 
This  cap  has  been  heated  over  a  Bunsen  burner  while  the  seal 


SEALING  IN.  125 

was  being  made.  By  this  means  the  seal  is  prevented  from 
cooling  quite  as  rapidly  as  it  would  if  the  cold  air  were  allowed 
free  access  to  it.  Some  glass-blowers  anneal  by  holding  the 
lamp  or  other  article  just  made  in  the  smoky  flame  from  the 
blow-pipe  with  the  air  blast  shut  off.  The  result  is  that  the 
glass  is  covered  with  a  coating  of  lamp-black,  which  is  sup- 
posed to  keep  it  warm  and  allow  it  to  cool  only  slowly.  It 
seems  highly  probable,  however,  that  it  has  precisely  the 
reverse  effect.  In  any  case  it  is  a  dirty  method,  and  one  not 
to  be  recommended. 

A  very  troublesome  occurrence  sometimes  takes  place  in  con- 
nection  with  sealing  in  the  filaments.  When  the  lamps  are  being 
exhausted  it  is  found  that  the  bulbs  are  spotted  all  about  on 
the  inside  with  dirty  white  spots,  and  they  are  in  consequence 
unfit  for  sale.  The  bulb  is  wasted,  though  the  filament  can 
be  taken  out  and  put  into  another  bulb.  The  appearance  of 
these  spots  is  not  altogether  easy  to  account  for,  as  they  only 
occur  occasionally.  Perhaps  during  one  day  a  number  of 
lamps  may  be  found  in  this  condition,  though  previously  they 
had  been  quite  free  from  it.  It  has  been  suggested  that  it  is 
caused  by  the  glass-blowers  chewing  tobacco,  but  the  Author 
has  found  it  to  occur  in  lamps  sealed  in  by  girls  who  resented 
the  imputation  of  chewing  tobacco.  The  spots  resemble  those 
which  may  often  be  seen  on  the  chimney  glasses  of  argand  gas 
burners,  and  there  seems  to  be  liitle  doubt  but  that  they  are 
due  to  a  similar  cause.  When  an  argand  gas  burner  is  first 
lighted,  drops  of  water,  produced  by  the  combustion  of  the  gas, 
condense  upon  the  cold  chimney.  This  water  soon  evaporates 
as  the  chimney  gets  warm,  but  before  it  has  done  so  it  has 
dissolved  certain  other  products  of  the  combustion  of  the  gas, 
and  these  it  leaves  behind  on  the  glass  in  the  form  of  spots 
exactly  where  the  drops  of  water  were  located.  When  an  argand 
burner  has  been  lighted  a  number  of  times  without  the  glass 
being  cleaned,  these  spots  are  very  apparent,  as  they  are  added 
to  at  each  time  of  lighting.  The  spots  on  the  inside  of  the 
lamp  bulbs  are  like  these  in  appearance,  and  are  probably  due 
to  a  similar  cause,  though  their  appearance  on  some  days  and 
not  on  others  is  somewhat  perplexing.  In  order  to  be  due  to 
the  same  cause  it  is,  of  course,  necessary  that  the  products  of 
combustion  of  the  gas  of  the  blow-pipe  flame  get  into  the  bulb, 


126          THE   INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

-which  can  easily  happen  during  the  sealing-in.  During  the 
sealing-in,  however,  the  bulb  is  too  warm  to  allow  of  any 
moisture  condensing  upon  it;  but,  nevertheless,  the  air  within 
it  is  extremely  moist,  partly  from  the  water  produced  by  the 
flame,  but  more  from  the  glass-blower  blowing  into  the  bulb 
so  many  times  during  the  process  of  sealing  in.  When  the 
sealing  in  is  done  and  the  lamp  cools  down,  the  moisture  will 
•condense  upon  the  inside  of  the  glass,  and  this  moisture  is 
ready  to  absorb  anything  there  may  be  within  the  bulb.  When 
this  moisture  is  driven  off,  either  by  again  heating  the  bulb 
or  exhausting  the  air,  the  solid  white  matter  is  left  behind  in 
the  form  of  spots  on  the  bulb,  precisely  as  it  is  in  the  case  of 
the  argand  chimneys.  The  question,  however,  arises,  why 
should  not  the  same  thing  always  happen.  When  it  does 
occur,  it  generally  does  so  with  the  work  of  several  glass- 
blowers  at  the  same  time.  Consequently,  the  cause  must  lie 
in  something  which  affects  them  all.  This  can  only  be  in  the 
quality  of  the  gas  itself,  or  of  the  air  blast.  It  may  be  that 
the  gas  on  some  days  contains  impurities  which  are  absent  on 
other  days.  The  air  blast  cannot  very  well  contain  anything 
unless  it  is  water.  Water  in  the  air  blast  may  possibly  effect 
the  combustion  of  the  gas. 

The  remedy  for  the  trouble  is  simple  enough.  The  bulbs 
must  not  be  allowed  to  cool  with  this  mixture  of  gases 
and  moisture  inside  them.  Instead  of  the  usual  annealing 
they  must  be  taken  as  soon  as  sealed  in,  and  placed  in  a 
hot  oven,  which  may  be  heated  with  steam  pipes,  until  they 
can  be  dealt  with.  They  are  then  taken  one  at  a  time  and 
exhausted  by  a  mechanical  air  pump,  an  operation  only  taking 
a  few  seconds.  The  lamps,  hot  from  the  oven,  are  connected 
by  rubber  tubes  through  a  three-way  cock,  to  a  vacuum  pump, 
and  exhausted,  and  then  refilled  with  pure  dry  air.  In  this 
way  no  moisture  is  allowed  to  condense  on  the  glass  and 
spots  are  entirely  prevented.  Such  precautions  are,  however, 
unnecessary,  unless  it  is  found  that  the  lamps  are  apt  to 
become  spotted.  The  process  of  mechanically  exhausting  the 
bulbs  after  sealing-in  and  re-filling  them  with  dry  air  is, 
however,  one  which  may  be  adopted  with  advantage  in  any 
case.  There  is  usually  a  film  of  moisture  on  the  inside  of  the 
bulb  after  sealing-in.  If  the  lamps  are  connected  to  the 


SEALING-IN.  1 27 

mercury  pumps  in  this  condition,  the  phosphoric  anhydride 
drying  tubes  will  require  very  much  more  frequent  renewal 
than  they  will  if  the  above  method  is  adopted. 

Before  leaving  the  subject  of  glass  blowing,  there  are  one  or 
two  points  in  connection  with  it  to  be  mentioned. 

Glass  tube  is  "  cut "  by  making  a  scratch  on  it  in  the 
direction  in  which  it  is  to  be  cut,  and  then  pulling  it  apart, 
and  at  the  same  time  pressing  the  scratched  part  outwards. 
In  this  way  it  will  usually  part  straight  across  and  per- 
fectly evenly.  The  scratch  may  be  made  with  the  edge 
of  a  file.  A  small,  fine,  flat  file  (not  a  three-cornered  one) 
answers  best.  After  a  while  the  corners  of  the  file  will  get 
blunt.  The  thin  edges  of  the  file  must  then  be  ground 
slightly,  and  the  four  cutting  edges  will  again  be  sharp.  A  file 
is  not  necessary.  Any  piece  of  hard  steel  ground  to  a  some- 
what sharp  edge  will  do.  A  razor  ground  so  that  it  is  just 
about  sharp  enough  to  cut  cheese  will  scratch  glass  very  well. 
A  properly  sharpened  razor  will  not  do  it. 

When  the  tube  is  too  thick,  or  its  situation  will  not  allow  it 
to  be  pulled  apart  after  the  scratching,  it  may  be  severed  by 
applying  to  the  scratch  a  small  globule  of  melting  glass, 
formed  at  the  end  of  a  piece  of  thin  drawn-out  glass,  such  as 
may  always  be  found  on  the  glass-blower's  table  after  he  has 
been  at  work  a  short  time.  This  will  start  a  crack,  and  the 
tube  can  then  be  severed  with  very  little  force.  In  order  that 
the  crack  shall  be  certain  of  going  in  the  right  direction  it  is 
advisable  to  scratch  the  glass  all  round  instead  of  only  for  a 
very  short  distance.  In  cutting  large  articles  like  the  glasses 
used  for  flashing-in,  a  crack  may  be  started  in  the  above-men- 
tioned way,  and  may  then  be  led  round  in  the  direction  re- 
quired by  holding  against  the  glass  a  hot  piece  of  metal,  like 
the  end  of  a  file  or  a  piece  of  wire,  slightly  in  advance  of  the 
end  of  the  crack,  which  will  rapidly  follow  it. 

There  are  other  methods  which  may  be  used  for  cutting 
such  large  articles.  For  instance,  a  string  may  be  tied  round 
the  article,  and  be  moistened  with  spirits  and  lighted.  If  a 
scratch  has  been  previously  made  at  some  point  under  the 
string,  a  crack  will  be  started  there,  and  will  probably  extend 
all  the  way  round  exactly  in  the  line  of  the  string. 


128         THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

The  glasses  used  for  flashing-in  have  to  be  ground  in 
order  that  they  may  be  perfectly  flat  on  the  ends,  or  it  will 
be  impossible  to  obtain  in  them  the  necessary  degree  of 
vacuum.  The  grinding  is  done  on  the  flat  side  of  a  grind- 
stone. The  "  stone  "  may  be  made  of  wood  or  lead.  An  iron 
shell  filled  with  lead  answers  very  well.  Fine  emery  powder 
with  water  is  used  on  the  lead.  The  glass  to  be  ground  is 
held  firmly  and  lightly  on  the  stone,  and  moved  about  to  and 
from  the  centre,  care  being  taken  to  keep  it  always  perfectly 
upright,  or  it  will  not  be  ground  flat.  It  must  be  ground 
until  the  whole  of  the  edge  has  come  into  contact  with  the 
stone.  When  ground  sufficiently,  the  edge  may  be  smoothed 
by  further  grinding  or  polishing  on  another  (wooden)  wheel, 
using  powdered  pumice  instead  of  emery. 

Stoppers,  taps,  and  valves  required  for  pumps  and  other 
apparatus  have  also  to  be  ground.  When  the  bearing,  or 
rubbing  surface,  of  the  two  parts  is  not  large,  as  in  the  case 
of  pump  valves,  the  two  parts  may  be  directly  ground  together, 
using  fine  emery  powder  and  water  or  turpentine.  The  glass 
valve  can  usually  be  welded  to  a  piece  of  glass  rod,  by  means 
of  which  it  can  be  ground  round  in  its  seat.  This  may  be 
done  by  hand,  but  it  is  much  quicker  to  fix  it  in  a  lathe,  and, 
if  the  parts  have  been  well  made,  very  little  grinding  is  neces- 
sary. On  a  glass-blowing  machine  it  is  possible  to  make 
valves  so  perfectly  circular  as  to  need  no  grinding  at  all.  Such 
a  valve  with  a  little  mercury  round  it  will  hold  a  vacuum 
without  leaking  against  the  full  pressure  of  the  atmosphere. 
Hand-made  valves,  on  the  contrary,  are  seldom,  if  ever,  so  per- 
fect but  that  the  mercury  is  speedily  forced  through  under 
the  same  circumstances,  and  they  consequently  have  to  be 
ground. 

The  two  parts  of  articles  like  stoppers  and  stop-cocks  which 
have  a  large  extent  of  bearing  surface  are  best  ground  sepa- 
rately in  the  first  instance  and  then  finally  together.  A 
hollow  cone  and  a  plug  to  fit  it,  of  the  shape  required  for  the 
stopper  or  cock,  are  made  out  of  copper,  and  are  used  to  grind 
the  glass  with  until  the  whole  of  the  surfaces  have  been  ground 
over.  The  two  glass  parts  are  then  finished  by  grinding  them 
together.  Care  must  be  taken  in  lathe  grinding  that  the  glass 
is  not  allowed  to  get  too  hot  or  it  may  crack.  Great  care 


SEALING-IN.  129 

must  also  be  taken  that  no  undue  strain  is  produced.  The 
part  turned  by  the  lathe  must  be  so  fixed  that  it  will  slip  if 
there  is  any  tendency  for  the  two  parts  to  jamb  together,  or 
else  the  other  part  must  be  so  loosely  held  that  in  case  of  it 
seizing  it  can  rotate  with  the  rest.  Glass  cocks  can  be  bought 
cheaply,  and  are  seldom  made  in  the  lamp  factories.  Those 
which  are  usually  sold,  however,  are  made  of  German  glass, 
which  has  no  lead  in  it.  It  is  difficult  to  join  this  glass  to  lead 
glass  satisfactorily,  although  it  can  be  done.  A  joint  between 
two  very  different  kinds  of  glass  is  always  liable  to  fail,  even 
though  it  appears  to  be  perfectly  welded. 


CHAPTER  XII. 


EXHAUSTING. 

THE  filament  having  been  sealed  into  the  bulb,  the  next 
process  is  that  of  exhausting  the  air.  This  process  is, 
perhaps,  the  most  important  of  all,  as  upon  the  excellence 
•of  the  vacuum  produced  depends,  to  a  very  great  extent,  the 
life  of  the  filament.  It  is,  moreover,  the  most  costly  per 
lamp  of  any  of  the  processes. 

It  is  necessary  that  the  air  be  removed  from  the  bulb, 
primarily,  on  account  of  the  combustible  nature  of  the 
filament.  If  a  carbon  filament  is  heated  to  incandescence 
in  air,  by  a  current  of  electricity,  it  rapidly  burns  away. 
Even  if  the  air  is  removed  as  far  as  possible  by  exhausting 
•with  a  good  mechanical  air-pump,  there  will  be  sufficient  left 
behind  in  the  bulb  to  burn  the  filament  enough  to  speedily 
<?ause  its  breakage.  It  is  necessary  to  produce  a  very  much 
better  vacuum  than  can  be  obtained  by  any  ordinary  mechanical 
vacuum  pump.  A  very  small  quantity  of  air  left  behind  in 
the  bulb  will  be  enough  to  burn  the  filament  slightly,  and 
thereby  shorten  its  life. 

It  has,  at  various  times,  been  proposed  to  fill  the  bulb  with 
nitrogen  gas,  and  then  to  only  partially  exhaust.  In  this 
way  a  residual  atmosphere  of  nitrogen  could  be  left  behind, 
which  would  be  incapable  of  burning  the  filament.  Another 
advantage  of  this  method  would  be  that  the  exhaustion,  not 
requiring  to  be  carried  to  a  very  high  state,  could  be  done 
very  quickly  with  a  mechanical  air-pump. 

There  are,  however,  several  reasons  why  this  method  cannot 
be  successfully  used.  The  combustion  of  the  filament  with 
the  oxygen,  in  the  case  of  a  residual  atmosphere  of  air,  is  not 

x  2 


132          THE    INCANDESCENT    LAMP   AND    ITS    MANUFACTURE. 

the  only  action  which  has  to  be  considered.  In  a  lamp  con- 
taining a  residual  atmosphere  of  air,  nitrogen,  or  any  gas,, 
there  are  convection  currents  set  up  in  the  gas  within  the  bulb,, 
which  consequently  becomes  very  hot.  The  gas  is  heated  by 
contact  with  the  white  hot  filament,  and  a  continual  circula- 
tion is  kept  up  from  the  filament  to  the  bulb,  and  back  again. 
The  bulb,  in  consequence,  becomes  so  hot  that  it  cannot  be 
touched,  and  it  will  char  wood  or  paper,  or  anything  of  a  com- 
bustible nature  with  which  it  may  be  in  contact.  One  of  the 
most  cherished  attributes  of  the  incandescent  lamp  is  that  it 
gives  off  very  little  heat,  and  is  perfectly  safe,  and  can  be 
handled  with  impunity  when  lighted,  and  may  be  placed  with 
safety  anywhere,  even  with  very  inflammable  material  in  close^ 
proximity.  All  these  advantages  are  lost  when  the  vacuum  is 
not  perfect.  No  residual  gas  can  be  allowed  in  the  bulb. 
Besides  making  the  lamp  very  hot,  there  are  other  objections 
to  the  residual  gas.  Unless  exactly  the  same  pressure  of  gas 
be  left  in  all  the  lamps,  they  will  vary  in  brightness,  even 
though  the  filaments  are  exactly  alike  in  every  particular. 
Lamps  with  a  greater  pressure  will  be  duller  than  those  with 
a  less.  The  heat  of  the  filament  will  be  carried  away  to  the- 
glass  by  the  currents  of  gas  more  quickly,  and  a  higher  voltage 
and  current  will  be  necessary  to  maintain  a  given  temperature, 
In  a  properly  exhausted  lamp  the  effect  of  convection  currents 
is  negligible,  because  there  are  practically  none.  But  with  a 
vacuum  below  the  proper  standard  convection  currents  begin, 
and  have  a  rapidly  increasing  effect  in  cooling  the  filament  r 
and  the  poorer  the  vacuum  the  more  power  must  be  supplied 
to  the  filament  to  maintain  the  temperature.  Consequently, 
at  a  given  temperature,  a  lamp  with  a  poor  vacuum  is  less 
efficient  than  one  with  a  good  vacuum  at  that  temperature. 
It  takes  more  power  to  produce  a  given  quantity  of  light. 
Such  a  lamp,  with  a  poor  vacuum,  can,  however,  be  run 
brighter  and  brighter  until  its  efficiency  in  watts  per  candle 
is  equal  to  that  of  the  properly  exhausted  lamp,  but  its  tem- 
perature is,  in  consequence,  very  much  higher. 

When  the  vacuum  is  not  good,  and  there  are  consequently 
convection  currents  set  up  within  the  bulb,  the  law  of  c  at  dl 
does  not  appear  to  apply,  or,  at  any  rate,  it  becomes  useless 
for  the  practical  purpose  of  calculating  the  sizes  of  filaments^ 


EXHAUSTING.  133 

The  actual  amount  of  the  effect  of  convection  depends  not 
only  on  the  pressure  of  the  gas  but  on  the  size  of  the  bulb. 
If  the  bulb  is  small,  the  gas  within  it  will  get  much  hotter,  and 
the  filament  will  also  be  hotter  than  if  the  bulb  is  large,  when 
there  is  a  greater  quantity. of  gas  to  be  heated  and  a  larger 
surface  of  glass  for  it  to  cool  against.  It,  therefore,  becomes 
necessary  that  the  vacuum  in  a  lamp  shall  be  so  far  perfect 
that  convection  currents  are  absent  altogether.  In  order  to 
produce  such  a  vacuum  it  is  necessary  to  use  what  are  known 
as  mercurial  air-pumps,  mechanical  air-pumps  not  being  good 
enough  for  the  purpose.  Of  course  it  is  impossible  to  obtain 
a  theoretically  perfect  vacuum,  but  with  the  mercurial  air- 
pump  such  a  degree  of  exhaustion  can  be  produced  as  to  reduce 
the  pressure  of  the  gas  to  something  unmeasureable,  and  to 
entirely  remove  all  signs  of  convection. 

The  mercurial  air-pump,  although  only  within  the  last  few 
years  brought  to  a  state  of  efficiency — it  cannot  yet  be  called 
perfection — is  by  no  means  a  recent  invention.  It  was  used 
more  than  two  hundred  years  ago.  In  the  seventeenth  cen- 
tury, Torricelli  showed  how  a  vacuum  could  be  produced  by 
filling  a  tube,  something  over  30in.  long  and  closed  at  one 
end,  with  mercury,  and  then  inverting  it,  while  temporarily 
closing  the  open  end,  and  placing  that  end  under  the  surface 
of  mercury  contained  in  a  cup.  The  mercury  in  the  tube  then 
descended  until  it  stood  at  what  is  now  called  the  "  baromet- 
rical height "  above  the  level  of  the  mercury  in  the  cup.  The 
enclosed  space  within  the  tube  above  the  top  of  the  mercury 
contained,  therefore,  a  vacuum,  as  in  an  ordinary  barometer. 
A  vacuum  so  produced  is  always  now  called  a  "  Torricellian  " 
vacuum. 

Progress  from  that  time  was  slow.  It  was  not  until  1865, 
when  Sprengel  brought  out  his  well-known  form  of  pump, 
that  high  vacua  were  easily  obtained,  although,  ten  years  pre- 
viously, Geissler  had  invented  the  pump  which  is  called  by 
his  name,  and  which,  in  a  modified  and  improved  form,  is 
now  found  to  be  equal,  if  not  superior,  to  the  Sprengel  pump. 

Anyone  who  wishes  for  full  information  on  the  development 
of  the  mercurial  air-pump  from  the  time  of  Torricelli  must 
study  the  lectures  delivered  in  1887,  before  the  Society  of 
Arts,  by  Prof.  S.  P.  Thompson.  A  reprint  of  these  lectures, 


134 


THE    INCANDESCENT    LAMF    AND    ITS    MANUFACTURE. 


from  the  Journal  of  that  Society,  is  published  by  Spon.  Any- 
one reading  these  lectures  will  understand  that  the  modern 
forms  of  pumps  are  in  no  case  the  invention  of  any  individual 
person,  but  are  the  outcome  of  improvements  made  by  a 
number  of  people  at  various  times. 

Prof.  Thompson  classifies  the  pumps  under  several  heads,, 
the  two  principal  ones  being  (1)  upward  driving  pumps,  and 
(2)  downward  driving  pumps.  Geissler's  pump  comes  under 
the  first  heading,  and  Sprengel's  under  the  second.  It  is  with 
these  two  forms  of  pump  in  some  shape  or  other  that  the 
lamp-maker  is  concerned. 

F 


FIG.  40. — Geissler  Pump. 

Geissler's  Pump. 

The  original  form  of  Geissler's  pump  is  shown  in  Fig.  40. 
A  is  a  large  bulb  (unless  otherwise  stated,  the  parts  of  the 
pumps  described  are  all  made  of  glass),  B  is  a  tube  some  3ft. 
long,  on  the  end  of  which  is  fastened  a  flexible  rubber  tube,  C, 
which  is  again  connected  to  the  lower  part  of  a  vessel,  D, 
which  is  also  open  at  the  top.  Above  the  bulb  A  is  a  cock,  E, 
by  means  of  which  the  bulb  A  can  be  connected,  either  to  the 
tubes  G  or  F,  but  not  to  both  at  the  same  time,  or  it  can  be 
shut  off  from  both.  The  vessel  to  be  exhausted  is  connected 
to  G.  F  is  open  to  the  air.  A  sufficient  quantity  of  mercury 
to  rather  more  than  fill  A,  B  and  C  is  poured  in  at  D.  The 
vessel  D  is  capable  of  being  raised  and  lowered. 


EXHAUSTING.  135 

The  action  is  as  follows: — Let  it  be  supposed  that  the 
vessel  D  is  raised  up  rather  higher  than  A,  as  shown  in  the 
figure.  The  bulb  A  is  consequently  filled  with  mercury,  while 
D  is  empty ;  the  cock  E  being  turned  so  that  A  is  connected 
with  the  outside  air  at  F.  This  cock  is  then  turned  so  as  to 
connect  A,  through  the  tube  G,  to  the  vessel  to  be  exhausted, 
the  air  in  which  is,  of  course,  at  this  stage,  at  atmospheric 
pressure.  D  is  then  lowered,  and  the  level  of  the  mercury  in 
A  is  lowered  in  consequence,  the  mercury  running  down  B 
and  through  C  into  D.  As  the  mercury  in  A  descends,  air  is 
drawn  from  the  vessel  to  be  exhausted  through  G  into  A,  so 
that,  when  the  mercury  has  descended  below  A,  the  whole 
space  is  filled  up  by  air  drawn  through  G.  This  air, 
having  expanded  from  the  closed  vessel  attached  to  G,  is,  of 
course,  at  a  less  pressure  than  that  of  the  atmosphere.  The 
cock  E  is  then  turned  so  as  to  cut  off  communication  between 
A  and  G.  D  is  then  slowly  raised,  and  the  mercury  flows 
gradually  back  into  A.  As  the  mercury  rises  in  A,  it  com- 
presses the  air  above  it.  When  it  has  got  some  distance  up, 
the  pressure  of  the  air  in  A  will  be  equal  to  the  atmospheric 
pressure.  When  this  is  the  case,  the  cock  E  should  be  turned, 
so  as  to  connect  A  with  the  outside  air  through  F.  The 
vessel  D  being  continually  raised,  the  mercury,  also  constantly 
rising  in  A,  drives  the  air  above  it  out  at  F  until  it  is  all 
expelled,  and  the  mercury  fills  up  the  whole  space  to  the  cock 
E,  which  is  then  turned  so  as  to  cut  off  all  communication 
between  A  and  F  or  G.  D  is  then  lowered  again,  but  the 
mercury  in  A  does  not  begin  to  descend  until  D  is  some  30in. 
below  A.  As  the  mercury  in  A  descends,  it  leaves  a  vacuous 
space  above  it ;  a  Torricellian  vacuum,  in  fact.  The  mercury 
having  entirely  run  out  of  A,  the  cock  E  is  again  turned,  so  as 
to  connect  A  through  G  to  the  vessel  being  exhausted.  The 
air  remaining  in  that  vessel  again  expands  and  fills  A.  The 
cock  E  is  again  turned  off,  and  D  is  raised,  the  mercury  again 
rising  in  A,  and  driving  the  air  before  it  until  it  is  let  out  at  F 
by  again  turning  the  cock.  By  successive  operations  in  this 
way  a  vacuum  is  gradually  produced  in  the  vessel  connected 
to  G,  that  proportion  of  the  air  which  expands  into  A  being 
taken  out  at  each  stroke.  A  pump  like  this  requires  very 
careful  working.  When  the  exhaustion  is  well  advanced, 


136          THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

great  care  must  be  used  in  lifting  the  vessel  D  gently,  or  the 
impact  of  the  mercury  against  the  glass,  when  it  rises  to  the 
top  of  A,  may  break  the  apparatus. 

When  the  vacuum  is  well  advanced,  the  mercury  in  A  will 
have  reached  the  top  when  its  level  in  D  is  30in.  below.  If 
connection  be  then  made  with  F,  there  will  be  an  in-rush  of 
air  through  F,  which  will  drive  the  mercury  down  again. 
Communication  with  the  air  through  F  must,  therefore,  not 
be  made  until  D  has  been  raised,  so  that  it  is  level  with  E. 
Communication  with  G  must  also  not  be  made  until  D  has 
been  again  lowered  30in.  below  A,  or  mercury  will  be  forced 
through  G  into  the  vessel  being  exhausted. 

There  is,  however,  one  great  objection  to  this  pattern  of 
pump.  Air  is  certain  to  leak  into  the  pump  part  to  stop- cock 
E.  It  is,  in  fact,  impossible  with  such  a  pump  to  produce  a 
vacuum  sufficiently  good  for  incandescent  lamps. 

In  1880  Lane-Fox  produced  a  modified  form  of  this  pump 
(Fig.  41). 

Instead  of  the  three-way  cock  above  the  bulb,  he  used  a 
stopper  F  to  connect  A  with  the  outside  air.  The  lamps  to 
be  exhausted  were  connected  by  the  tube  G  below  the  pump 
bulb  instead  of  above  it,  the  tube  G  being  carried  upwards 
more  than  30in.  above  the  stopper,  to  H  and  down  to  K.  The 
stopper  F  was  sealed  by  mercury  in  a  cup  formed  immediately 
above  the  seat  of  the  stopper.  This  form  of  pump  is  better 
than  the  one  first  described,  on  account  of  having  a  direct 
communication,  without  any  stop-cock,  to  the  vessel  to  be 
exhausted,  and  by  the  fact  that  the  stopper  F  could  be  sealed 
with  mercury.  It  is,  however,  extremely  difficult  to  obtain  a 
high  degree  of  vacuum  with  a  pump  opening  directly  to  the 
air  in  this  way. 

The  method  of  working  this  pump  is  similar  to  the  last. 
When  the  lamps  to  be  exhausted  have  been  connected  at  K, 
the  vessel  D,  being  full  of  mercury,  is  lifted,  the  stopper  F 
being  raised.  The  mercury  rising  in  A  drives  the  air  before 
it  and  out  at  F.  When  the  mercury  has  reached  to  the 
height  L  in  the  cup  above  the  seat  of  the  stopper  F,  the 
stopper  F  is  inserted,  so  as  to  retain  some  of  the  mercury  in 
the  cup  to  help  to  keep  it  air-tight.  The  level  of  the  mercury 
in  D  is  then  the  same  as  in  L.  The  mercury  has  also  risen  in 


EXHAUSTING, 


137 


the  tube  H,  but  not  to  the  height  of  L,  because  the  air  which  it 
drives  up  the  tube  G,  H,  K,  is  unable  to  get  out.  D  is  now  gently 
lowered.  The  mercury  in  A  would  not  descend  until  D  was 
80in.  below  it  but  for  the  fact  of  the  connection  to  the  lamps 
at  G.  When,  however,  D  has  been  lowered  only  a  short  dis- 
tance, and  before  the  mercury  in  A  has  parted  contact  with 


FIG.  41. — Lane-Fox  Pump. 

the  stopper  F,  the  air  in  K,  H,  G  runs  round  into  A,  and 
bubbles  up  through  the  mercury.  Great  care  is  necessary  in 
lowering  D  steadily  and  slowly,  or  the  rush  of  air  from  G 
through  the  mercury  may  cause  sufficient  commotion  to  break 
the  glass.  D  is  gradually  lowered  until  the  mercury  is  all  out 
of  A.  The  operation  is  then  repeated  a  number  of  times, 


138         THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


until  the  pump  ceases  to  take  any  more  air  out.  After  the 
first  half-dozen  strokes  or  so  the  mercury  rises  in  G,  H,  so  as 
to  be  about  30in.  above  the  level  in  D.  Hence  the  necessity 
for  carrying  H  more  than  30in.  above  L,  to  prevent  the  mer- 
cury from  running  round  into  K. 

A  very  much  better  pump  than  either  of  the  foregoing  is 
one  known  as  Toepler's  (Fig.  42),  which,  curiously  enough, 


FIG.  42.— Toepler  Pump. 

was  invented  before  the  Sprengel  pump.  Toepler's  pump,  if 
properly  made,  is  capable  of  producing  as  good  a  vacuum  as 
the  Sprengel  pump.  It  is  like  the  pump  last  described,  except 
that,  instead  of  the  bulb  A  opening  to  the  air  through  a  stopper, 
there  is  a  tube  M  of  small  bore  joined  on  and  carried  down- 
wards rather  more  than  30in.,  and  terminating  near  the 
bottom  of  a  cup,  N. 


EXHAUSTING.  139 

The  great  advantage  of  this  form  of  pump  over  those  already 
described  lies  in  the  fact  of  the  stopper,  or  cock  connecting 
the  pump  bulb  with  the  outside  air,  being  entirely  done  away 
with.  As  the  vessel  D  is  raised,  the  mercury  rises  in  A,  and 
drives  the  air  before  it  through  the  tube  M  and  out  at  N ;  D  is 
raised  high  enough  for  some  of  the  mercury  in  A  to  flow 
through  M  into  N,  and  so  drive  all  the  air  out  at  N.  In  this 
way  a  little  mercury  passes  at  each  stroke  into  N.  When 
the  depth  of  mercury,  thus  carried  into  N,  is  about  an  inch 
above  the  end  of  the  tube  M,  all  further  amount  carried  into 
N  overflows  into  the  cup  P,  from  which  it  is  poured  back 
into  D. 

The  air  having  thus  been  all  expelled  through  N,  the  tube  M 
and  the  bottom  of  the  cup  N  being  full  of  mercury,  the  vessel 
D  is  lowered.  The  mercury  in  the  tube  M  consequently  parts 
somewhere  about  the  point  marked  0,  one  part  descending 
in  the  tube  M  and  the  other  part  in  the  bulb  A.  The  mercury 
in  the  tube  M  will  then  stand  at  the  barometrical  height  above 
the  level  in  N.  The  height  of  the  mercury  in  this  tube  gives, 
therefore,  an  indication  of  the  degree  of  exhaustion  in  the  bulb 
A,  and  when  the  mercury  has  descended  in  A  below  the  tube 
G,  the  height  in  M  gives  also  an  indication  of  the  degree  of 
vacuum  in  the  lamps. 

The  inside  diameter  of  the  tube  M  must  be  small,  so  that 
the  descending  mercury  entirely  fills  it.  When  the  exhaustion 
is  only  in  the  first  stages,  the  mercury  rising  in  A  will  force  the 
air  above  it  into  the  tube  M,  thereby  driving  the  column  of  mer- 
cury in  M  down.  When  the  mercury  is  thus  driven  entirely 
out  of  M,  the  air  following  it  will  bubble  up  and  escape  through 
the  mercury  hi  N.  This  will  begin  to  happen  before  the  mer- 
cury rising  in  the  pump  has  arrived  at  the  top  of  the  pump 
bulb  A.  Each  time  the  mercury  is  raised,  less  air  remains  to 
be  pumped  out.  After  a  while,  therefore,  the  mercury  ascend- 
ing in  A  will  enter  the  narrow  tube  M,  before  the  column  in 
M  has  begun  to  descend.  The  column  in  M  may  only  begin 
to  descend  when  the  mercury  passing  through  A  has  arrived 
within,  say,  Gin.  of  it.  All  the  air  being  taken  out  at 
that  stroke  is,  therefore,  contained  in  Gin.  length  of  the 
small  tube,  and  this  Gin.  length  of  air  is  under  only  a  very 
slight  pressure  at  the  top  of  the  tube.  As  it  is  driven  lower 


140         THE    INCANDESCENT    LAMP   AND    ITS    MANUFACTURE. 

down,  it  is  gradually  compressed,  and  becomes  shorter  and 
shorter,  so  that,  by  the  time  it  reaches  the  bottom  of  the  tube 
M,  it  is,  perhaps,  only  an  inch  long,  and  it  has  become  com- 
pressed to  atmospheric  pressure  by  the  time  it  escapes  through 
the  mercury  in  N. 

After  a  while,  when  the  exhaustion  is  still  better,  there  will 
be,  perhaps,  only  lin.  length  of  air  between  the  top  of  the 
mercury  in  M  and  that  coming  from  A  at  the  moment  when 
the  column  in  M  begins  to  descend.  If  this  inch  of  air  is 
watched  closely  as  it  descends,  it  is  seen,  as  before,  to  get 
shorter  and  shorter,  and  it  may  very  likely  be  seen  to  disappear 
altogether  before  it  reaches  the  outlet.  This  total  disappear- 
ance of  the  air,  before  it  gets  to  the  outlet,  may  sometimes 
be  due  to  its  getting  compressed  to  such  a  small  size  that  it 
is  no  longer  large  enough  to  fill  the  bore  of  the  tube.  The 
small  bubble  thus  produced  may  be  swept  down  the  back  part 
of  the  tube  and  therefore  escape  without  being  noticed.  Often, 
however,  it  is  certain  that  no  such  small  bubble  of  air  passes 
out  at  all,  but  the  peculiarity  is  due  to  the  fact  that  some  of 
the  air  sticks  to  the  sides  of  the  glass  tube  all  the  way  down. 
Consequently,  when  there  is  only  a  very  small  quantity  at  the 
top  of  the  tube,  there  may  be  none  at  all  left  to  pass  out  at 
the  bottom,  it  having  all  adhered  to  the  glass,  where  it 
remains  as  a  film  between  the  tube  and  the  mercury.  This 
is  the  great  defect  in  this  type  of  pump.  By  keeping  on 
pumping  long  enough,  however,  the  air  is  gradually  swept  off 
the  tube,  and  a  good  vacuum  can  be  obtained. 

Care  must  be  used  in  raising  the  mercury,  as  in  the  other 
two  pumps  described,  so  as  not  to  bring  it  violently  into 
contact  with  the  top  of  the  pump.  It  must  also  be  lowered 
with  care  at  starting,  on  account  of  the  rush  of  air  from  the 
lamps  into  the  pump,  as  in  the  Lane-Fox  pump. 

Fig.  43  shows  a  method  of  overcoming  this  last  trouble. 
The  tube  G,  leading  from  the  lamps,  instead  of  entering  the 
tube  B  directly,  is  led  into  another  tube  P  which  opens  into 
the  pump  just  above  and  just  below  the  bulb  A.  Therefore, 
when  the  mercury  is  descending  in  A,  the  air  from  the  lamps 
passes  from  G,  up  P,  and  so  enters  the  bulb  A  at  the  top  in- 
stead of  below  the  mercury.  Thus  the  large  quantity  of  mer- 
cury in  A  is  not  violently  shaken  up. 


EXHAUSTING. 


141 


Sometimes  a  valve  is  used  in  G  to  prevent  the  mercury 
from  rising  in  that  tube.  The  long  extension  of  G  is,  there- 
fore, unnecessary.  There  is,  however,  no  great  advantage  in 
such  an  arrangement,  and  the  cost  of  the  valve  is  much 
greater  than  that  of  the  extra  length  of  tube. 

A  great  improvement  in  this  class  of  pump  is  obtained  when 
the  air  is  expelled  by  the  mercury  into  an  already  partially 
exhausted  chamber,  instead  of  directly  into  the  air.  An 
excellent  pump  of  this  kind  (Fig.  44),  was  designed  by  Mr.  J. 
Swinburne.  A  small  bore  tube,  E,  is  joined  to  the  top  of  the 
pump  bulb  A,  as  in  Toepler's  pump.  This  tube  is  bent  over 
in  the  manner  shown,  and  opens  into  a  small  bulb  F,  above 
which  there  is  a  valve  K. 


Bl  | 

FIG.  43.— Safety  Side  Tube  Arrangement  for  Use  with  Geisaler  Class  of  Puinp. 

The  pump,  with  the  lamps  attached  at  L,  is  first  exhausted 
as  far  as  possible  by  a  mechanical  air-pump,  the  connection  to 
which  is  made  at  H.  As  this  exhaustion  proceeds,  the  mercury 
rises  in  the  shaft  B,  until  it  reaches  nearly  up  to  the  point 
where  the  tube  G  enters  the  shaft  B.  Its  height  is  then,  if 
the  mechanical  pump  is  a  good  one,  about  30in.  above 
the  level  in  the  reservoir  D.  The  pump  and  the  lamps  con- 
nected to  it  are,  therefore,  exhausted,  as  far  as  the  mechanical 
pump  is  capable,  before  the  mercury  is  used. 

The  reservoir  D  is  now  raised,  and  the  mercury  rises  in  A 
and  G,  driving  the  air  before  it  through  E  and  F,  past  the 
valve  K,  and  out  at  H.  D  is  raised  high  enough  to  carry  the 


142 


THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


mercury  into  the  valve  chamber.  The  valve  floats  upon  the 
mercury.  D  is  then  lowered,  and  the  mercury  in  the  valve 
•chamber  descends,  but  the  valve  is  heavy  enough  to  reach  its 
seat  and  close  the  outlet  before  all  the  mercury  has  run  out  of 
the  valve  chamber.  The  valve  is,  therefore,  sealed  by  a  ring  of 
mercury  in  the  same  way  as  the  stopper  in  the  Lane-Fox 


<s 


FIG.  44. — Swinburne  Pump. 

pump.  On  the  mercury  descending  further  it  leaves  a  very 
perfect  vacuum  below  the  valve.  The  small  tube  E,  being 
shaped  as  shown  in  the  illustration,  retains  the  last  portion  of 
the  descending  mercury,  and  thereby  forms  another  seal  or 
valve. 

As  the  mercury  descends  in  A  there  is  no  rush  of  air  from 
O.     The  mercury  descending  in  G  will  be  nearly  on  a  level 


EXHAUSTING.  143 

with  that  in  A,  even  at  the  first  stroke,  provided  that  the 
preliminary  exhaustion  by  the  mechanical  pump  was  good. 
Consequently,  when  the  exhaustion  is  begun  in  this  way,  with 
a  mechanical  pump,  there  is  no  need  for  the  extra  side  tube  P, 
Fig.  43. 

One  great  advantage  of  exhausting  into  an  already  partially 
exhausted  chamber  is  that  the  mercury  reservoir  D  does  not 
require  lifiing  so  high.  If  the  chamber  K  were  open  to  the 
air  this  reservoir  would  have  to  be  raised  up  to  the  level  of  K, 
in  order  to  lift  the  valve.  When,  however,  a  good  mechanical 
vacuum  is  maintained  in  K,  it  will  only  require  to  be  raised 
to  a  height  about  30in.  below  K,  and  when  the  reservoir  is 
down,  the  level  of  the  mercury  in  it  will  be  about  30in. 
below  the  junction  of  G  with  B.  It  will  only  require  to  be 
lifted  so  that  the  level  of  the  mercury  in  it  is  raised  by  a 
height  equal  to  the  distance  of  the  valve  chamber  above  the 
junction  of  G  with  B.  This  will  be  only  about  15in.  or  so 
instead  of  45in.,  if  the  mechanical  vacuum  were  not  used. 

In  working  this  pump,  when  the  exhaustion  is  well  advanced 
it  is  not  necessary  to  drive  the  mercury  into  the  valve  chamber 
at  every  stroke.  When  there  is  a  very  little  air  being  taken 
off  at  each  stroke,  the  amount  of  it  can  be  seen  by  the  space  it 
occupies  in  the  tube  E  as  it  is  driven  along  between  the 
mercury  remaining  in  that  tube  and  that  which  rises  from  A. 
When  the  amount  carried  up  at  each  stroke  is  small,  it  is 
sufficient  to  drive  it  into  the  small  bulb  F  only,  where  it  will  be 
shut  in  by  the  mercury  seal  left  in  the  tube  E.  A  dozen  strokes 
or  so  of  the  pump  may  be  made,  each  time  putting  a 
little  air  into  F  before  the  pressure  in  F  is  sufficient  to  break 
the  seal  in  E.  When  there  are  signs  that  this  is  going  to 
occur  the  mercury  must  be  raised  higher,  so  that  the  air  con- 
tained in  F  is  driven  past  the  valve. 

This  pump  will  give  a  very  good  vacuum  indeed,  but  it  is 
more  suitable  for  the  laboratory  than  for  the  factory,  as  it 
must  be  handled  very  carefully  in  order  that  the  rising 
mercury  shall  not  break  the  tube  E.  For  factory  use,  Mr. 
Swinburne  dispenses  with  the  tube  altogether,  the  valve  and 
the  bulb  F  being  then  in  a  line  with  the  centre  of  the  pump 
bulb  A.  The  pump  will  then  safely  withstand  the  rougher 
usage  of  the  factory,  and  it  will,  if  properly  worked,  give  an 


144         THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

equally  good  vacuum.  The  Author  has  used  this  type  of  pump 
a  good  deal  for  factory  work,  and  it  will  be  further  described 
when  dealing  with  pumps  as  arranged  for  the  factory. 

TJie  Sprengel  Pump. 

The  illustration  (Fig.  45)  shows  the  simple  form  of  the 
Sprengel  pump.  A  is  a  funnel  below  which  is  a  stop-cock,  C. 
B  is  a  small  bore  tube,  called  the  "shaft,"  or  the  " fall-tube." 
G  is  a  tube  leading  from  near  the  top  of  the  shaft  to  the  vessel 
to  be  exhausted.  The  tube  B  terminates  within  a  short  dis- 
tance of  the  bottom  of  a  vessel  D,  having  a  spout  F,  over  the 


cO 


FIG.  45.— Sprengel  Pump.     Simple  Form. 

cup  E.  The  length  of  B  from  its  junction  with  G  to  the  level 
of  F  should  be  at  least  three  feet.  Mercury  is  poured  into  A, 
whence  it  flows  downwards  into  B.  The  rate  of  flow  is 
adjusted  by  the  cock  C,  so  that  a  very  small  stream  is  formed. 
The  result  is  that  air  is  drawn  from  G  and  carried  down  the 
tube  B.  The  falling  mercury  breaks  up  into  short  lengths, 
entrapping  small  columns  of  air  at  the  juncture  of  G 
and  B.  The  weight  of  the  mercury  forces  these  little 
columns  of  air  down  the  shaft  B,  and  they  escape  at 
the  surface  of  the  mercury  in  D,  while  the  mercury  itself 
runs  out  through  F  into  the  cup  E,  from  which  it  is  every 
now  and  then  poured  back  again  into  A.  This  operation 


EXHAUSTING.  145 

is  kept  continually  going  until  the  mercury  ceases  to  carry 
down  any  more  air.  It  then  falls  into  the  tube  B  with  a  sharp 
rattling  noise,  showing  that  there  is  not  enough  air  between 
the  drops  of  the  mercury  to  act  as  a  cushion.  It  does  not* 
however,  follow  that  because  the  mercury  rattles  the  vacuum 
is  necessarily  good.  It  will  rattle  while  still  taking  down 
small  bubbles  of  air.  Similar  action  to  that  mentioned  in  the 
case  of  Toepler's  pump  is  also  going  on  here.  Some  of  the  air 
entrapped  by  the  mercury  sticks  to  the  fall  tube,  and  the  mer- 
cury goes  down,  leaving  it  behind.  If,  however,  the  pump  is 
kept  going  for  some  time  after  it  appears  to  be  taking  down  no 
more  air,  the  vacuum  will  be  found  to  improve,  showing  that 
the  air  is  gradually  swept  off  the  glass. 


FIG.  46.— Sprengel  Pump  Distributor  for  Six  Fall  Tubed. 

This  simple  form  of  Sprengel  pump,  although  better  than 
the  simple  form  of  Geissler,  is  not  adapted  for  lamp  factory 
requirements  as  it  stands.  One  reason  is  that  it  is  very  slow. 
In  order  to  increase  the  speed,  several  fall  tubes  may  be  con- 
nected together.  Thus,  if  there  are  six  tubes,  the  time  taken 
to  produce  a  vacuum  such  that  the  mercury  rattles  will  be 
about  one-sixth  of  the  time  which  would  be  taken  by  a  single 
tube.  From  this  point,  however,  the  time  taken  to  produce  a 
good  vacuum  will  not  be  nearly  so  much  reduced.  The  total 
time  of  pumping  is,  nevertheless,  very  considerably  shortened. 

One  great  trouble  with  the  Sprengel  pump  is  that  the  fall- 
tubes  crack,  often  after  being  only  a  very  short  time  in  use, 
owing  to  the  continual  hammering  of  the  mercury  in  the  tube. 

A  method  of  connecting  together  several  fall-tubes  is  shown 
in  Fig.  46.  The  tube  G,  communicating  with  the  lamps,  is 
connected  to  the  upper  side  of  a  tube  K  K,  about  one  inch  in 

L 


146 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


diameter.  On  the  lower  side  of  K  are  a  number  of  cups  or 
funnels,  C  C,  according  to  the  number  of  fall-tubes  required. 
Through  one  end  of  K  passes  a  smaller  tube  A,  having  a  small 
hole  D,  immediately  over  each  of  the  funnels  C  C.  The  whole 
of  this  arrangement  is  welded  together  so  as  to  be  perfectly 
air-tight.  The  fall-tubes  may  also  be  welded  to  the  funnel 
pieces  C  C,  but,  as  they  are  always  liable  to  break  after  a 
while  and,  consequently,  have  to  be  replaced,  it  is  more 
convenient  to  connect  them  by  such  an  attachment  as  is  shown 
on  the  right-hand  funnel  in  the  illustration.  E  is  a  rubber 
tube,  fitting  tightly  on  the  top  of  the  fall-tube  B,  and  on  the 
tail  of  the  funnel  C.  In  order  to  insure  that  this  connection 
shall  be  perfectly  air-tight,  it  is  entirely  covered  up  with 
mercury  held  in  the  cup  H.  This  cup  is  supported  by 
the  rubber  cork  F,  fitting  tightly  between  the  base  of  the 
cup  and  the  fall-tube.  The  cup  and  its  supporting  rubber 


FIG.  47.— Sprengel  Pump  Collector  for  Six  Fall  Tubes. 

can  be  moved  up  or  down  the  fall-tube,  so  that  the  rubber 
connection  E  can  easily  be  got  at  when  it  is  necessary  to 
change  a  fall-tube.  A  cracked  fall-tube  can  thus  be  replaced 
by  a  new  one  in  about  two  minutes.  When  the  pump  is  at 
work,  the  mercury  enters  through  the  tube  A,  and  forms  a 
small  stream  at  each  of  the  holes  DD.  These  streams  of 
mercury  falling  into  the  funnel  pieces  C  C,  carry  the  air  down 
the  fall-tubes  in  the  manner  already  described. 

The  fall-tubes  may  conveniently  terminate  at  the  bottom  in 
an  arrangement  such  as  shown  in  Fig.  47.  B  B  are  the  fall- 
tubes,  which  reach  nearly  to  the  bottom  of  the  horizontal  tube 
L,  into  which  they  pass  through  the  rubber  corks  0  0.  The 
mercury  coming  down  the  fall-tubes  flows  out  through  M, 
while  the  air  which  it  carries  down  escapes  through  N. 

The  method  of  exhausting  into  a  partial  vacuum,  as  already 
described  in  connection  with  the  Giessler  form  of  pump, 


EXHAUSTING.  147 

can  also  be  applied  to  the  Sprengel  pump  with  the  correspond- 
ing advantages.  The  length  of  the  fall-tubes,  and,  conse- 
quently, the  height  to  which  the  mercury  must  be  lifted,  can 
be  very  much  reduced,  while  the  air  is  more  readily  swept 
down  and  out  of  the  fall-tubes. 

In  all  the  pumps  hitherto  described  it  will  be  noticed  that 
the  mercury  has  to  be  lifted  to  the  top  of  the  pump  by  hand. 
As  mercury  is  very  heavy,  and  the  lifting  must  be  kept  up  con- 
tinuously, the  operation  is  a  very  tedious  one.  Various 
methods  of  doing  this  part  of  the  work  mechanically  and 
automatically  have,  in  consequence,  been  devised,  so  that  the 
pumps  when  once  started  may  be  left  to  take  care  of  them- 
selves until  a  good  vacuum  is  obtained. 

The  mercury  is  sometimes  pumped  up  from  the  lower 
reservoir  to  the  upper  one  by  an  ordinary  kind  of  force-pump, 
or  it  is  lifted  in  small  buckets  running  on  an  endless  band, 
after  the  manner  of  a  mud  dredger. 

Mercury  should,  however,  never  be  directly  exposed  to  the 
air  in  the  pump  room.  It  should  be  entirely  closed  up: 
firstly,  because  it  will  otherwise  get  dirty  ;  and,  secondly, 
because  it  is  very  poisonous  when  present  in  the  air  in  a  fine 
state  of  division  or  as  vapour.  Serious  cases  of  mercury 
poisoning  have  occurred  in  pump  rooms  where  the  mercury 
has  not  been  enclosed.  As  several  of  the  best  patterns  of 
pumps  of  both  types  entirely  enclose  the  mercury,  there  is  no 
excuse  for  this.  The  floor  of  the  pump-room  should  be  so 
constructed  that  any  mercury  which  may  be  accidently  spilt 
by  the  breaking  of  a  pump  or  otherwise  can  be  entirely  and 
quickly  gathered  up. 

The  most  convenient  way  of  lifting  the  mercury  is  by  air 
pressure.  It  may  be  accomplished  by  atmospheric  pressure 
forcing  the  mercury  into  an  exhausted  chamber,  where  a 
vacuum  is  maintained  by  a  mechanical  pump  driven  by  power. 
The  limit  to  the  height  to  which  the  mercury  ca.n  be  raised  in 
this  way,  hi  a  continuous  column,  is,  of  course,  30in.  If 
the  pump  is  one  which  requires  the  mercury  to  be  raised  a 
greater  height  than  this,  it  may  be  accomplished  by  lifting  it 
in  several  stages.  The  mercury  is  first  raised  to  one  exhausted 
chamber,  and  from  that  up  to  a  second  one  by  admitting  air  into 
the  first.  In  this  way  it  may  be  lifted  to  any  required  height. 

L  2 


148         THE   INCANDESCENT   LAMP   AND    ITS   MANUFACTURE. 

Another  method  is  to  use  compressed  air,  which,  according 
to  the  degree  of  pressure  used,  can  lift  the  mercury  in  one 
stage  to  any  desired  height. 

Mercury  can,  however,  be  lifted  into  a  vacuous  chamber  to 
a  much  greater  height  than  30in.  by  atmospheric  pressure 
alone,  provided  that  it  is  not  in  a  continuous  column.  The 
column  must  be  split  up  into  lengths  with  air  spaces  between, 
exactly,  in  fact,  like  a  Sprengel  pump  working  upwards.  The 
mercury  can,  in  this  way,  be  raised  to  any  height,  provided 
that  the  sum  of  the  vertical  heights  of  the  broken-up  columns 
in  the  tube  does  not  exceed  30in.  This  is  a  slow  method, 
as  the  diameter  of  the  lifting  tubes  must  necessarily  be  small 
in  order  that  the  air  may  drive  the  plugs  of  mercury  upwards 
without  bubbling  through  them.  The  number  of  lifting- tubes 
required  will  be  as  great  as  or  greater  than  the  number  of 
fall- tubes  of  the  pump. 

When  pumps  are  worked  by  hand,  it  is  impossible  for  one 
person  to  keep  more  than  two  or  three  going  at  the  same  time. 
By  automatic  methods  of  raising  the  mercury,  a  number  can, 
however,  be  attended  to  simultaneously.  It  is,  in  fact, 
essential  for  the  proper  working  of  the  lamp  factory  that  some 
other  means  than  hand  power  be  utilised.  The  illustration 
(Fig.  48)  shows  an  arrangement  adapted  to  a  Sprengel  pump 
for  lifting  the  mercury  by  compressed  air.  The  pump  is  a 
full-length  Sprengel.  The  lamps  are  connected  to  N.  The 
mercury  in  the  vessel  K  (corresponding  to  the  funnel  A, 
Fig.  45)  descends  by  gravity,  through  the  tube  M,  into  the 
pump-head  A,  and  down  the  fall-tubes  B  into  C,  carrying,  of 
course,  air,  drawn  from  the  lamps,  with  it.  The  air  thus 
carried  down  escapes  through  D,  while  the  mercury  falls 
through  E  and  F,  past  the  valve  G,  and  up  into  the  chamber 
H.  The  valve  G  is  capable  only  of  closing  the  tube  below  it 
and  not  above  it.  The  chamber  H  is  connected,  through  I, 
alternately  to  an  air-pump  and  to  the  atmosphere. 

As  the  mercury  falls,  it  gradually  collects  in  the  chamber  H 
until  it  nearly  fills  it,  H  being  open  to  the  atmosphere  through 
I.  Air  under  pressure  is  then  admitted  through  I,  and  presses 
on  the  mercury,  with  the  result  that  the  valve  G  closes  and  the 
mercury  in  H  is  forced  up  the  tube  J,  and  out  at  the  top  into 
K.  The  pump  in  the  meantime  continues  working.  The 


EXHAUSTING. 


149 


mercury  entering  C  from  the  fall- tubes  flows  out  through  E, 
and  collects  in  the  bulb  F  and  in  the  tube  below.  When  most 
of  the  mercury  has  been  driven  from  the  vessel  H  up  into  K, 
the  air  pressure  is  released,  and  H  is  again  open  to  the  atmo- 
sphere through  the  tube  I.  The  mercury  then  filling  the  tube 
J  falls  back  into  H,  the  reservoir  K  being  open  to  the  atmo- 
sphere at  the  top.  The  head  of  mercury  in  the  bulb  F  will 


FIG.  48.— Sprengel  Pump  with  Air-Pressure  Lift 

now  be  sufficient  to  lift  the  valve  G,  and  it  will  flow  into  H. 
In  a  few  minutes  H  will  again  be  nearly  filled  with  mercury ; 
air  pressure  will  be  applied  through  I,  and  a  further  supply 
of  mercury  will  be  lifted  to  K.  The  circulation  of  mercury  is 
thus  constantly  maintained,  and  the  pump  is  kept  working 
continuously. 

The   air  pressure,    intermittently    applied   at   I,  is   care- 
fully  maintained  at  a  fixed  value,   a  blow-off  valve  being 


150 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


arranged  so  that  it  cannot  exceed  the  proper  amount.  The 
pressure  is  such  that  it  can  support  a  column  of  mercury 
about  lin.  shorter  than  the  tube  J  ;  thus  it  is  never  able  to 
quite  empty  H  of  mercury  and  blow  air  up  the  tube  J.  The 
cock  for  turning  the  pressure  on  and  off  at  I  may  be  worked 
uutomatically — in  fact,  it  must  be  if  one  man  is  to  attend  to  a 
number  of  pumps.  One  large  mercury-lifting  apparatus  may, 
however,  be  arranged  to  supply  a  number  of  pumps. 

Fig.  49  shows  a  similar  method  applied  to  the  shortened 
form  of  Sprengel  pump.     The  mercury  is  raised,  by  means  of 


FIG.  49. — Shortened  Fcrm  of  Sprengel  Pump. 

alternate  applications  of  vacuum  and  air  at  the  normal  pres- 
sure, into  a  vacuous  chamber.  The  fall-tubes  E  R  may  be  quite 
short — 12in.  or  15in.  will  answer  very  well.  They  terminate 
at  the  bottom  in  the  chamber  S,  into  which  they  pass  through 
"air-tight  rubber  corks.  The  tube  T  connects  S  with  the  top 
of  the  bulb  U.  Below  U  is  a  valve  chamber  D  and  a  valve  C. 
This  valve  stops  the  flow  of  mercury  upwards  but  not  down- 
wards. The  tube  E  connects  the  bottom  of  the  valve-chamber 
with  a  tube  F,  leading  into  the  bottom  of  the  bulb  A.  The 
tube  G,  rising  out  of  A,  terminates  in  the  reservoir  K.  The 
tube  B,  inside  G,  is  open  at  both  ends  and  rises  to  near  the 
top  of  K.  At  X  it  passes  through  a  rubber  cork,  which  effec- 


,  EXHAUSTING.  151 

tually  closes  the  passage  from  G  into  K.  The  tube  H  connects 
with  G  just  below  the  cork  X.  The  tube  W  has  two  branches — 
one  branch,  Y,  enters  the  reservoir  K  through  an  air-tight 
rubber  cork,  and  the  other  branch,  V,  leads  to  the  top  of  the 
bulb  U.  The  tube  L  leads  from  the  bottom  of  the  reservoir 
K  to  the  vessel  M,  into  which  passes  the  tube  N,  which  carries 
the  mercury  into  the  pump-head  0.  The  lamps  are  connected 
to  P. 

The  action  of  this  apparatus  is  as  follows  : — The  lamps  being 
sealed  on  and  the  bulb  A  being  full  of  mercury,  a  vacuum, 
constantly  maintained  by  a  mechanical  pump,  is  applied  at  W, 
H  being  open  to  the  air.  The  first  result  is  that  air  is  drawn 
out  of  the  whole  apparatus,  including  the  lamps  connected  to 
P,  through  the  tubes  V  and  Y,  and  out  at  W.  The  mercury 
in  A  at  the  same  time  tends  to  rise  up  into  U,  and  also  up  the 
tube  B.  It  is,  however,  prevented  from  rising  into  U  by  the 
action  of  the  valve  C,  and  can,  therefore,  only  ascend  in  the 
tube  B,  which  it  is  driven  to  do  by  the  atmospheric  pressure 
upon  it  in  A,  communicated  through  the  outer  tube  G,  which 
is  in  connection  with  the  atmosphere  through  H.  The  top  of 
the  tube  B  is  turned  over  at  J,  so  that  the  mercury  flowing 
up  is  not  driven  into  Y,  but  is  turned  aside  and  falls  into  the 
bulb  K.  From  K  it  runs  by  gravity  into  M,  and  thence  up  N 
and  down  the  fall- tubes  K  R  into  S,  carrying  with  it  air,  which 
it  draws  from  the  lamps  through  P.  From  S,  both  the  mer- 
cury and  the  air,  which  it  has  carried  down,  rise  through  the 
tube  T  and  enter  the  top  of  the  bulb  U.  Here  the  mercury 
collects  and  the  air  passes  up  through  V  and  out  at  W.  The 
bulb  U  is  thus  gradually  filled  up  with  mercury,  which  is 
unable  to  escape  downwards,  as  the  valve  C  is  kept  up  by  the 
pressure  below  it.  This  pressure,  being  somewhat  greater 
than  that  of  the  column  of  mercury  in  B,  is,  of  course,  able  to 
hold  up  the  valve  C  against  the  lesser  pressure  of  the  head  of 
mercury  in  U. 

When  the  bulb  U  is  thus  nearly  filled  with  mercury  and  the 
bulb  A  is  nearly  empty,  a  vacuum,  produced  by  a  second 
mechanical  pump,  is  applied  at  H.  The  pressure  on  the 
mercury  remaining  in  A  is,  therefore,  removed,  and  the  mer- 
cury filling  the  tube  B  at  that  moment  falls  back  into  A.  The 
head  of  mercury  in  U  at  the  same  time  causes  the  valve  C  to 


152         THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

drop,  and  the  mercury  falls  by  gravity  from  U  through  the 
tube  E  into  the  bulb  A,  the  air  pressure  in  U  and  A  being 
about  equal,  as  both  of  these  chambers  are  at  the  reduced 
pressure  of  the  mechanical  vacuum  pumps  connected  to  H 
and  W. 

The  mercury  which  had  collected  in  U  having  thus  run 
into  A,  air  at  normal  pressure  is  again  admitted  at  H,  the 
valve  C  again  closes,  and  the  mercury  is  once  more  driven 
from  A  up  the  tube  B  into  the  reservoir  K,  from  which  it 
continues  to  flow  to  the  fall-tubes. 

The  action  of  the  cock  in  H,  not  shown  in  the  illustration, 
which  turns  on  alternately  vacuum  and  air  at  normal  pressure, 
is  timed  so  that  a  fresh  supply  of  mercury  is  lifted  into  K 
before  the  previous  quantity  has  all  run  out  into  the  fall-tubes. 
The  action  of  the  pump  is,  therefore,  continuous.  The  vessel 
M  forms  an  air-trap  to  prevent  any  air  from  being  carried  into 
N,  and  so  spoiling  the  vacuum  in  0. 

It  will  be  noticed  by  comparing  the  illustrations  that  the 
shortened  form  of  Sprengel  pump,  Fig.  49,  is  precisely  similar 
to,  and  works  in  the  same  manner  as,  the  long  form,  Fig.  48. 
In  the  long  form  the  mercury  is  lifted  by  air  above  atmospheric 
pressure  into  a  chamber  at  atmospheric  pressure,  the  air  drawn 
from  the  lamps  escaping  into  the  atmosphere  direct.  In  the 
shortened  form  the  mercury  is  raised  by  air  at  atmospheric 
pressure  into  a  chamber  below  atmospheric  pressure,  while  the 
air  drawn  from  the  lamps  escapes  through  a  vacuous  chamber. 

The  length  of  the  tube  B,  from  J  to  the  bottom  of  the  bulb 
A,  should  be  a  little  over  30in.,  so  that  under  no  circum- 
stances can  air,  entering  through  H,  find  its  way  either  into 
K  or  into  the  valve  chamber  D.  This  is  on  the  supposition 
that  a  good  mechanical  vacuum  is  maintained  at  W.  The 
pump  may,  however,  be  so  much  shortened  that  the  full  lift 
of  30in.  is  not  required.  The  length  of  the  tubes  G  and  J 
may,  therefore,  be  less,  but  the  vacuum  at  W  must  also  be 
correspondingly  lower. 

The  shortened  form  of  Sprengel  pump  is  not  exempt  from 
the  trouble  of  the  cracking  of  the  fall-tubes.  Although  the 
tubes  are  very  much  shorter  than  in  the  long  form,  the  actual 
height  of  the  fall  of  the  drops  of  mercury  upon  the  mercury 
columns  in  the  tubes  is  about  the  same  in  each  case. 


EXHAUSTING. 


153 


Similar  methods  for  lifting  the  mercury  can  be  applied  to 
the  Geissler  form  of  pump.  Fig.  50  shows  an  arrangement 
by  which  the  mercury  is  raised  by  the  alternate  application  of 
vacuum  and  air  at  normal  pressure.  This  form  of  pump  was 
first  used  by  Mr.  Weston.  A  is  the  pump  bulb,  B  is  a  reser- 
voir somewhat  larger.  These  two  bulbs  are  connected  by  the 
tube  C.  D  is  a  cock  for  drawing  off  the  mercury,  if  necessary, 
for  cleaning,  &c.  A  mechanical  exhaust  pump  is  applied  at 
K,  and  another  one  is  intermittently  applied  at  E.  The  lamps 


FIG.  50.— Weston  Pump. 

are  connected  to  H.  G  is  a  valve  for  preventing  the  mercury 
rising  into  H.  The  pump  contains  sufficient  mercury  to  fill 
the  bulb  B  and  the  tube  C  to  the  same  height. 

In  starting  the  pump,  vacuum  is  applied  at  both  K  and  E. 
Air  is  consequently  drawn  out  of  the  pump  and  the  lamps. 
When  the  mechanical  exhaustion  has  proceeded  as  far  as 
possible,  an  automatic  arrangement  cuts  off  the  vacuum  at  E 
and  admits  air  at  the  normal  pressure  ;  the  mercury  is  conse- 
quently forced  from  B  into  A,  sweeping  the  air  in  A  before  it. 
The  mercury  will  rise  to  L,  about  30in.  above  the  then  level 
in  B.  Mercury  also  flows  up  the  tube  F  as  far  as  the  valve 
G,  which  prevents  it  going  any  higher  in  that  direction. 


154          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

Vacuum  is  then  again  applied  at  E,  and  the  mercury  conse- 
quently falls  back  again  from  A  into  B,  leaving  behind,  how- 
ever, enough  to  form  a  seal  in  the  bent  tube  M.  The  mercury 
in  the  tube  F  also  falls  into  C,  the  valve  G  drops,  and  con- 
nection is  again  established  between  the  lamps  and  the  bulb 
A.  The  cycle  is  again  and  again  repeated,  until  a  good 
vacuum  is  obtained  in  the  lamps. 


FIG.  51. — Sawyer-Mann  Pump  Room. 

Fig.  51  gives  a  view  of  the  pump-room  of  the  Sawyer- 
Mann  lamp  factory  in  New  York,  and  is  reproduced  from 
the  New  York  Electrical  Eevieiv.  It  will  be  noticed  that  the 
pumps  are  of  a  similar  construction  to  the  one  just  described. 
In  the  foreground  are  seen  the  gauges  for  indicating  the 
degree  of  pressure  in  the  tubes  connected  to  the  mechanical 
pumps.  Great  care  has  to  be  taken  that  the  mechanical 
pumps  are  working  properly  all  the  time,  and  the  gauges  must 
be  constantly  watched. 

It  is  not  necessary  to  have  a  very  good  vacuum  at  E,  as  the 
mercury  is  not  required  to  be  lowered  by  30in.  from  L.  The 


EXHAUSTING. 


155 


height  from  F  to  L  need  not  be  more  than  half  that  amount, 
so  that  a  vacuum  of  a  little  over  15in.  will  suffice  to  draw  the 
mercury  below  F.  The  pump  is,  however,  sometimes  repre- 
sented with  the  bulb  B  only  just  below  the  level  of  A.  In 
this  case  the  vacuum  at  E  must  be  greater,  or  the  mercury 
will  not  descend  below  F.  The  distance  between  M  and  L 
must  also  then  be  increased,  or  else  the  full  pressure  of  the 
atmosphere  must  not  be  admitted  at  E. 


FIG.  52. — Swinburne  Pump.     Factory  Pattern. 

Instead  of  applying  alternate  vacuum  and  air  at  atmo- 
spheric pressure  at  E,  alternate  applications  of  air  above 
atmospheric  pressure  and  air  at  atmospheric  pressure  may  be 
used.  In  this  case  the  bulb  B  must  be  situate  more  than 
30in.  below  F. 

For  factory  use,  Mr.  Swinburne  worked  his  form  of  pump 
in  a  similar  way.  Air  under  pressure  was  used  to  raise  the 
mercury.  Fig.  52  shows  the  arrangement.  The  pump  itself 


156          THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 

has  been  already  described  on  page  141.  The  shaft  B  passes 
through  a  rubber  cork  L,  into  the  bottle  M,  to  within  a  very 
little  of  the  bottom.  A  tube,  N,  also  passes  through  the  rubber 


FIG.  53.  —View  of  a  Pump-Room  with  Swinburne  Style  of  Pump. 

cork.  In  starting  the  pump  the  cock  0  is  turned,  thereby  con- 
necting the  pump  with  the  mechanical  vacuum  pump.  Air  is 
therefore  drawn  out  of  the  pump  and  the  lamps,  and  mercury 
from  the  bottle  M  rises  in  the  shaft  B  until  it  forms  a  column 


EXHAUSTING. 


157 


nearly  30in.  high,  reaching  to  a  point  a  little  below  the  junc- 
tion of  the  tube  G  with  the  shaft  B.  Air  under  pressure  is 
then  admitted  at  N.  The  mercury  is  consequently  forced  from 
M  up  the  shaft  B,  filling  the  bulbs  A  and  F,  until  it  reaches 
the  valve  chamber  K,  driving  the  air  before  it.  At  this  point, 
the  full  air  pressure  being  exerted,  the  mercury  rises  no 
higher.  The  air  pressure  required  is  not  great — only  about 
61b.  or  71b.  to  the  square  inch,  representing  a  rise  of  the  mer- 
cury column  from  G  to  K  of  12in.  or  14in.  By  an  automatic 
arrangement,  the  pressure  is  then  taken  off,  the  tube  N  being 
connected  with  the  outside  air.  The  mercury  consequently 


THE  ClfCTRICHH. 


Fia.  54. — Details  of  Top  of  Mercury  Reservoir  of  Pumps  in  Fig.  53. 

descends,  the  valve  K  closing  and  remaining  sealed,  with  some 
mercury  left  behind  in  the  valve  chamber.  The  mercury  drops 
to  below  G ;  the  air  left  in  the  lamps  therefore  expands  into 
A,  and  the  operation  is  again  repeated. 

Fig.  53  gives  a  view  of  part  of  a  pump-room,  with  this 
type  of  pump,  erected  by  the  Author.  The  pumps  are  of  a 
rather  large  size,  containing  nearly  301b.  of  mercury  each. 
They  differ  from  that  shown  in  Fig.  52  in  some  of  the  details. 
For  instance,  instead  of  the  rubber  cork  L  in  the  reservoir 
M,  the  arrangement  shown  in  Fig.  54  was  adopted.  A  is  a 
hard  wood  cap,  fitting  the  top  of  the  jar  B,  and  having  two 


158          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

metal  tubes,  C  and  D,  passing  through  it.  These  tubes  are 
screwed  in  so  as  to  be  quite  air-tight.  E  is  the  shaft  of  the 
pump,  passing  through  D  to  the  bottom  of  the  jar  B.  F  is  a 
piece  of  rubber  tube,  bound  tightly  with  wire  round  D  and  E, 
thereby  making  an  air-tight  joint.  G  is  a  metal  collar  round 
the  neck  of  the  jar,  immediately  under  the  rim  L.  H  is  an 
upright  strip  of  metal  fixed  to  G,  there  being  another  similar 
one  on  the  farther  side.  J  is  a  metal  cross-bar,  extending  at 
each  end  into  slots  in  the  uprights  H.  K  is  a  screw  passing 
through  J.  A  rubber  washer  is  placed  between  the  cap  A  and 
the  top  of  the  jar.  The  screw  K  is  tightened  until  the  cap  is 
pressed  down,  so  that  the  joint  is  air-tight.  The  air  pressure 
pipe  is  connected  to  C.  This  arrangement  was  found  to  be 
much  more  reliable  than  the  simple  rubber  cork,  owing  to  the 
latter  not  unfrequently  working  loose  and  allowing  the  air  to 
escape. 

In  the  illustration  there  are  four  rows  of  pumps,  two 
rows  back  to  back,  with  a  passage  between  sufficiently 
wide  to  allow  of  replacing  or  repairing  a  pump  from  behind. 
The  horizontal  pipe  at  the  top  is  the  mechanical  vacuum  pipe, 
having  a  branch  to  each  pump.  Every  pump  has  a  glass 
stop-cock  at  the  top,  by  means  of  which  it  can  be  connected 
to  or  cut  off  from  the  mechanical  vacuum  pump.  The  pipe 
just  below  the  level  of  the  table  supplies  the  gas  for  heating 
the  lamps.  A  few  inches  below  this  is  the  air-pressure  pipe. 
Both  of  these  pipes  have  a  branch  with  a  stop-cock  for  each 
pump. 

Another  modification  of  the  Geissler  pump  adapted  for 
factory  work  is  represented  in  Fig.  55.  It  was  designed  by 
Mr.  Rankin  Kennedy,  and  used  at  the  Woodside  Electric 
Works,  Glasgow.  It  will  be  seen  that  this  pump  is  a  shortened 
form  of  Toepler's  pump  (p.  138).  A  is  the  pump  bulb,  B  the 
reservoir,  Y  is  a  small  bore  tube  leading  from  the  top  of  the 
pump  bulb  into  a  mercury  seal  C  (corresponding  to  the  tube 
O  M  and  the  mercury  seal  N,  respectively,  in  Fig.  42). 

The  arrangement  shown  is  worked  by  a  vacuum,  produced 
by  a  mechanical  pump,  applied  at  K,  with  alternate  applica- 
tions of  vacuum  and  air  at  atmospheric  pressure  at  0,  in  the 
same  manner  as  in  the  Weston  pump  (p.  153).  The  shaft  T 
passes  to  nearly  the  bottom  of  the  reservoir  B,  and  is  of  such 


EXHAUSTING. 


159 


a  length  that  the  height  of  the  pump  from  the  top  of  the  tube 
Y  to  the  bottom  of  the  bulbous  part  of  the  reservoir  B  is  less 
than  30in.,  though  it  is  carried  downwards  into  the  narrow 
extension  of  the  reservoir  preferably  to  a  greater  distance 
than  30in.  from  Y,  in  order  that  the  air  pressure  may  under 
no  circumstances  be  able  to  reach  the  end  and  blow  through. 


FIG.  55. — Kennedy  Pump. 

In  starting  the  pump, 'the  mercury  being  in  the  reservoir 
B,  as  represented  in  the  diagram,  a  mechanical  vacuum  is 
applied  through  K,  by  turning  the  cock  F.  The  cock  G  is 
closed  and  the  three-way  cock  0  connects  B  with  the  atmo- 
sphere. Air  is,  therefore,  drawn  from  the  bulb  A  and  the 
lamps  through  the  mercury  seal  C  and  out  at  K.  The 
mercury  in  B  rises  in  T  and  fills  the  pump  bulb  A  and 
overflows  into  C.  The  cock  0  is  then  turned,  cutting  off  B 


160         THE   INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

from  the  atmosphere  and  connecting  it  to  a  mechanical 
vacuum.  The  pressure  being  thus  removed  from  the  surface 
of  the  mercury  in  B,  the  mercury  descends  from  A  back  into 
B,  leaving  a  Torricellian  vacuum  in  A  secured  by  the  seal  C. 
Air  from  the  lamps  then  expands  and  fills  A,  when  the  cycle 
is  again  repeated,  the  mercury  sweeping  the  air  from  A  past 
the  seal.  The  excess  of  mercury  which  passes  into  C  at  each 
stroke  is  returned  to  the  reservoir  B  when  under  vacuum  by 
opening  the  cock  G. 

By  increasing  the  length  of  the  shaft  T,  so  that  the  height 
from  the  point  X,  where  the  tube  E  joins  the  shaft  T,  to 
the  bottom  of  the  bulbous  part  of  the  reservoir  B,  is  slightly 
over  30in.,  this  pump  may  be  worked  by  compressed  air  and 
air  at  atmospheric  pressure  exactly  as  described  in  connec- 
tion with  the  Swinburne  pump  (p.  155).  This  pump  was 
described  in  the  Electrical  Review,  Vol.  XXXIII.,  p.  391. 

Thus  far,  each  different  pump,  and  the  way  it  works, 
has  been  described  separately.  There  are,  however,  certain 
matters  to  be  considered  in  connection  with  the  process  of 
exhausting,  which  apply  equally  to  all  forms  of  pump. 

An  American  lamp-maker  likens  the  process  of  exhausting 
air  from  a  bulb  to  driving  flies  from  a  room  containing  several 
millions  of  them.  "  They  leave,"  he  says,  "by  great  droves 
at  first,  but  the  real  labour  begins  when  only  a  few  remain. 
The  task  is  usually  called  complete  when  all  except  the  last 
dozen  have  found  the  means  of  exit."  This  is  not  a  bad  simile. 
It  is  the  last  part  of  the  exhaustion  which  gives  the  trouble. 

A  little  calculation  upon  the  Geissler  form  of  pump,  with, 
say,  the  capacity  of  the  pump  bulb  equal  to  that  of  the  lamps 
being  pumped,  shows  that  it  should  not  take  very  many 
strokes  in  order  to  reduce  the  amount  of  air  in  the  lamps  to 
something  very  close  indeed  to  zero.  In  practice,  however,  it 
takes  very  considerably  longer  than  the  calculation  would 
lead  one  to  expect.  The  reason  of  this  is  twofold.  Firstly, 
the  carbon  filaments  of  the  lamps  occlude  gas,  which  takes 
time  to  come  out ;  and,  secondly,  the  air  has  a  way  of  sticking 
to  the  glass,  both  of  the  pumps  and  the  lamps.  The  occluded 
gases  are  removed  by  lighting  up  the  lamps,  when  the  exhaus- 
tion has  proceeded  far  enough  to  make  it  safe  to  do  so.  The 
current  should  not  be  turned  on  until  the  vacuum  in  the 


EXHAUSTING.  161 

lamps  appears  to  be  as  good  as  can  be  obtained.  The  filaments 
are  brought  to  a  dull  red  heat  only,  at  first ;  and  gradually, 
as  the  gas  driven  out  is  pumped  away,  they  are  made  brighter 
and  brighter  until  eventually  they  are  brought  up  to  a  state 
of  incandescence  equal,  at  least,  to  that  at  which  they  will  be 
run  as  finished  lamps.  When  the  current  is  first  turned  on, 
it  will  at  once  be  seen  that  gas  is  driven  from  the  filaments 
by  the  pump  again  taking  out  air.  After  the  full  brightness 
has  been  reached,  the  pumping  must  be  continued  until  the 
vacuum  is  again  as  perfect  as  possible. 

The  way  in  which  the  current  is  applied  may  be  seen  in 
Fig.  53.  Flexible  wires  having  small  hooks  or  loops  on  the 
end  are  hung  on  the  platinum  wires  of  the  lamps.  Below  the 
table  is  seen  a  resistance  and  switch  for  regulating  the  current. 
Lighting  up  the  lamps  during  pumping  in  this  way  necessi- 
tates, of  course,  dynamos  and  engine  and  boiler  power  propor- 
tional to  the  number  of  lamps  to  be  run.  As,  however,  the 
current  is  not  applied  until  there  is  a  good  vacuum  in  the 
lamps,  it  is  best,  if  possible,  to  arrange  that  some  of  the  lamps 
shall  be  under  current,  whilst  the  others  are  in  the  first  stages 
of  exhaustion.  In  this  way  a  smaller  generating  plant  will 
suffice  than  would  be  required  if  all  the  lamps  were  to  be 
lighted  at  the  same  time.  It  is,  however,  a  somewhat  difficult 
matter  to  arrange  this  in  practice.  The  question  of  power  is 
a  serious  one.  If  all  the  lamps  are  ready  for  current  at  about 
the  same  time  there  will  be,  perhaps,  during  the  day,  periods 
of  an  hour  when  no  current  at  all  is  required,  while  half 
an  hour  afterwards  the  full  power  may  be  called  for.  This 
necessitates  a  large  plant  and  boilers  which  will  make  steam 
very  rapidly.  If  there  is  not  sufficient  steam  ready  when  it  is 
required  a  delay  will  be  caused,  which  may  prevent  a  whole 
round  of  lamps  from  being  finished  in  time  for  sealing  off 
before  closing  time.  It  is,  therefore,  much  better  to  use 
secondary  batteries,  which  can  be  charged  continuously  by  a 
much  smaller  plant  than  would  be  required  for  direct  work- 
ing, the  batteries  being  of  a  size  large  enough  to  supply  the 
whole  of  the  lamps  at  one  time  if  required. 

With  regard  to  the  gases  which  come  from  the  filament 
when  it  is  thus  heated  by  the  current,  those  which  come  off 
at  the  temperature  of  red  heat,  and  below,  are,  no  doubt, 

M 


162          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

rightly  termed  "  occluded  " — that  is  to  say,  they  have  been 
absorbed  by  the  filament  since  carbonisation.  Those,  however, 
which  make  their  appearance  when  the  filaments  are  bright 
are  more  probably  products  of  the  more  complete  carbonisa- 
tion of  the  filaments  by  the  high  temperature  to  which  they 
are  subjected  by  the  current. 

The  other  action,  which  prolongs  the  time  required  to  per- 
fect the  vacuum  beyond  the  theoretical  period,  that  of  the 
air  sticking  to  the  glass  of  the  pump  and  the  lamps,  is  more 
difficult  to  deal  with.  This  air,  which  appears  to  adhere  as  a 
film  to  the  glass,  will  come  off  gradually,  but  the  process  is  a 
very  slow  one;  in  order  to  assist  it  the  lamps  are  heated 
during  the  pumping.  It  is  not,  however,  an  easy  matter  to 
heat  the  pumps,  and  it  is  seldom  attempted,  except  in  the 
laboratory.  In  Fig.  53  a  method  of  heating  the  lamps  can 
be  seen.  A  metal  cover  is  lowered  over  the  lamps,  and  a 
Bunsen  gas  flame  is  applied  below.  The  lamps  can  thus  be 
heated  to  any  desired  extent  up  to  the  softening  points  of  the 
glass. 

In  some  factories  the  lamps  are  attached  to  the  pumps  the 
other  way  up,  with  the  glass  tube,  connecting  them  with  the 
pump,  passing  through  a  hole  in  the  top  of  the  cover.  This 
arrangement  is  not  so  good,  as  the  lamps  cannot  be  so  well 
heated  when  the  hot  air  is  allowed  to  escape  through  the  top 
of  the  cover. 

When  the  lamps  are  heated,  the  air  is  driven  off  the  glass 
and  is  pumped  out.  Fortunately  for  the  lamp-maker,  the 
removal  of  the  air  film  from  the  pump  itself,  and  from  the 
tube  leading  to  the  lamps,  is  of  no  very  great  consequence,  for 
the  reason  that  it  takes  such  a  long  time  to  come  off.  It  is 
not  necessary  to  make  elaborate  arrangements  for  heating  the 
pumps.  If  a  pump,  being  perfectly  air-tight  and  in  good 
order,  be  worked  until  no  more  air  is  taken  out,  and  be  then 
left  to  itself  for  24  hours,  and  after  that  interval  be  started 
again,  it  will  be  found  to  again  take  out  air,  this  air  having 
come  off  the  surface  of  the  glass.  It  takes  some  hours  for 
an  appreciable  quantity  of  air  to  accumulate  in  this  way. 

Although,  then,  when  the  pump  ceases  to  take  out  any  more 
.air,  and  a  film  of  air  is  still  adhering  to  the  glass,  there 
may  be,  nevertheless,  an  exceedingly  good  vacuum  within  the 


EXHAUSTING.  163 

pump  and  the  tubes  and  the  lamps,  the  lamps  themselves, 
having  been  heated,  do  not  contain  an  air  film,  while  the  air 
film  on  the  glass  of  the  pump  and  the  tubes  takes  a  long 
while  getting  into  the  vacuous  space.  The  lamps  can,  there- 
fore, be  sealed  off  before  the  air  film  has  had  time  to  come  off 
the  glass  of  the  pump  and  spoil  the  vacuum.  The  vacuum  in 
the  lamps  may,  consequently,  be  just  as  good  as  though  the 
pumping  had  been  going  on  for  some  days,  gradually  getting 
rid  of  the  air  film  in  the  pump. 

When  the  lamps  are  sealed  off,  air  is  let  into  the  pump 
before  another  lot  of  lamps  is  sealed  on.     On  account  of  this 


FIG.  56. — Swinburne's  Arrangement  for  Constantly  Maintaining  a  Vacuum 

in  a  Pump. 

the  air  film  in  the  pump  is  renewed  with  each  batch  of  lamps, 
and  is  never  got  rid  of.  At  the  British  Association  meeting 
in  1890,  Mr.  Swinburne  described  a  method  for  sealing  on 
fresh  lamps  without  letting  the  full  atmospheric  pressure  of 
air  into  the  pump.  Fig.  56  shows  the  arrangement.  The 
tube  A  leads  to  the  lamps,  and  the  tube  G  to  the  pump. 
There  is  a  vessel  of  mercury  in  connection  with  the  bottom  of 
the  tube  D,  so  that  when  there  is  a  vacuum  in  the  pump  and 
the  lamps  there  is  a  column  of  mercury  in  D  reaching  to  the 
point  C.  When  a  batch  of  lamps  is  sealed  off,  and  another 
lot  is  to  be  sealed  on,  the  mercury  reservoir  below  D  is  lifted 
sufficiently  to  cause  the  mercury  in  D  to  rise  and  fill  the 

M2 


164  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

bulb  B,  and,  to  an  equal  height,  the  tube  E.  A  cock  in  the 
lower  part  of  D  is  then  turned,  thereby  cutting  off  communica- 
cation  with  the  movable  mercury  reservoir.  Air  is  then  let 
into  the  apparatus  at  the  point  where  the  lamps  are  to  be 
sealed  on.  The  result  is  that  the  air  entering  through  A 
drives  the  mercury  from  B  up  the  tube  E  F,  where  it  will 
stand  at  the  barometrical  height  above  the  level  in  B.  The 
lamps  are  now  sealed  on,  and  are  exhausted  through  a  tube 
connected  to  A  by  a  mechanical  pump.  The  mercury  column 
in  E  F  therefore  descends  until  it  is  nearly  level  with  that  in 
B.  The  connection  between  A  and  the  mechanical  pump 
is  then  sealed  up,  the  cock  in  D  is  again  opened,  and  the 
mercury  reservoir  is  lowered.  The  mercury,  therefore,  runs 
out  of  B  and  E  to  about  its  former  height,  C,  in  the  tube  D. 
The  passage  is  then  open  between  the  new  lot  of  lamps  and 
the  pump,  which  is  then  set  going.  In  this  way  the  greatest 
pressure  of  air  at  any  time  allowed  within  the  pump  can  be 
kept  down  to  that  equal  to  about  half-an-inch  of  mercury.  Such, 
a  contrivance  would,  however,  be  of  little  practical  use  in  con- 
nection with  most  forms  of  pumps,  because  there  is  another 
channel  by  which  the  air-film  in  the  pump  is  continually 
maintained. 

The  mercury,  in  all  the  pumps  described,  and  parts  of  the 
interior  of  the  pumps  themselves,  are,  at  intervals,  in  contact 
with  air  either  at  or  above  atmospheric  pressure.  The  mercury 
is  forced  from  these  high-pressure  parts  of  the  pumps  into- 
those  parts  where  the  perfect  vacuum  is  required,  and  from 
which  it  is  expected  to  remove  all  the  air.  The  very  mercury 
itself,  however,  in  its  passage  through  the  tubes  from  the  high- 
pressure  to  the  low-pressure  regions,  sweeps  a  film  of  air 
along  those  tubes  from  the  high-pressure  parts  to  the  low- 
pressure  chamber.  Thus  the  mercury  itself  maintains  a  small 
supply  of  air  in  the  very  chamber  which  it  is  required  to 
exhaust.  The  amount  of  air  introduced  in  this  way  may  be, 
in  some  forms  of  pumps,  extremely  small,  but  in  others  it  is 
very  perceptible,  and,  of  course,  puts  a  limit  to  the  perfection 
of  vacuum  obtainable. 

In  the  case  of  the  Geissler  form  of  pump,  either  as  in  Fig.  50 
or  52,  the  action  proceeds  in  this  way :  When  the  mercury  is 
filling  the  pump  bulb  A,  the  reservoirs  B  or  M  respectively 


EXHAUSTING.  165 

a-re  nearly  empty  of  mercury,  and  experience  the  full  air- 
pressure.  As  soon  as  this  air-pressure  is  released  the  mercury 
flows  back  into  the  reservoir,  imprisoning  a  film  of  air  between 
itself  and  the  glass  walls  of  the  reservoir  and  (in  the  case  of 
Fig.  53)  the  lower  end  of  the  pump  shaft.  When  the  mercury 
next  flows  from  the  reservoir  into  the  pump  it  carries  part  of 
this  film  along  with  it,  gradually  working  it  forward.  At  each 
stroke  of  the  pump  a  little  of  it  is  delivered  into  the  pump 
head  and  a  little  more  is  taken  in  at  the  bottom  of  the  shaft 
to  be  gradually  worked  in.  The  amount  of  air  thus  introduced 
is  less  with  the  form  of  pump  reservoir  shown  in  Fig.  52  than 
with  that  of  Fig.  50.  In  the  latter  case  the  film  over  the 
whole  surface  of  the  reservoir  is  gradually  worked  forward, 
while  in  the  former  arrangement  only  that  on  the  part  of  the 
pump  shaft  which  extends  into  the  reservoir  can  enter  the 
pump,  the  film  on  the  reservoir  itself  having  no  suitable  con- 
nection with  the  pump  shaft. 

In  the  Sprengel  pump  (Fig.  48)  the  same  action  goes  on  in 
the  reservoir  K.  The  mercury  here,  rising  and  falling,  works 
the  air  film  into  the  tube  M,  and  so  into  the  pump  head.  The 
shortened  form  of  pump  (Fig.  49)  is,  however,  much  better  in 
this  respect.  All  the  while  this  pump  is  working  there  is  a 
vacuum  maintained  in  the  reservoir  K  by  a  mechanical  pump. 
If  this  vacuum  were  always  maintained,  no  air  film  to  speak 
of  could  be  formed  in  K  ;  it  is,  however,  lost  every  time  a 
fresh  lot  of  lamps  is  put  on  the  pumps,  unless  a  method  like 
Fig.  56  is  used.  The  vacuum  in  this  part  would  probably 
be  lost  in  any  event  during  the  night  when  the  mechanical 
pump  had  ceased  working.  An  air  film  will  thus  be  formed  in 
K,  and  gradually  work  into  M.  Little  of  it  will,  however, 
find  its  way  into  the  tube  N.  The  primary  object  of  the  trap 
M  N  is  to  catch  bubbles  of  air  carried  there  by  the  mercury, 
and  prevent  them  from  getting  into  the  pump  head,  such 
bubbles  being  formed  by  the  splashing  of  the  mercury  falling 
from  J  into  the  vessel  K.  It,  however,  serves  the  double  pur- 
pose of  stopping  the  progress  of  air  bubbles,  and  to  a  great 
•extent  of  the  air  film. 

That  a  film  of  air  really  does  exist  between  the  mercury  and 
the  glass  may  be  readily  ascertained  by  heating  the  shaft  or 
other  tube  of  a  pump  containing  mercury  with  a  Bunsen  gas 


166          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

flarne.  As  soon  as  the  heat  gets  through  the  glass,  bubbles 
appear  on  the  inside,  and  grow  larger  and  larger  as  the  heat- 
ing is  continued,  until  they  rise  through  the  mercury  and 
escape  at  the  top  of  the  column.  If  these  bubbles  were  mer- 
cury vapour  or  moisture,  they  would  be  condensed  in  passing 
up  through  the  cold  mercury  column,  and  would  not  reach 
the  top,  whereas  they  pass  up  before  the  upper  part  of  the 
mercury  column  is  sensibly  heated,  and  escape  at  the  surface. 

It  is  well  known  that  mercurial  barometers  have  to  be  boiled 
for  a  similar  reason,  in  order  to  expel  this  air. 

A  method  of  overcoming  this  trouble  in  the  case  of  the 
Geissler  form  of  pump  has  been  contemplated  by  the  Author. 
It  consists  in  enclosing  the  mercury  in  the  jar  or  reservoir 
at  the  base  of  the  pump  in  a  rubber  bag,  something  like  the 
"  bladder  "  of  a  foot-ball,  the  neck  of  the  rubber  being  tied 
over  the  end  of  the  shaft.  The  mercury  would  then  be 
raised  in  the  same  manner  by  compressed  air,  with  the 
difference  that  the  air  does  not  come  into  contact  with  the 
mercury  or  the  end  of  the  shaft.  Consequently,  after  the 
pump  has  been  worked  a  few  times,  and  the  original  air  film 
has  been  worked  out,  no  further  air  film  can  enter  by  the 
bottom  of  the  pump,  and  a  method  for  always  keeping  a 
-vacuum  in  the  pump,  like  that  in  Fig.  56,  might  be  used 
with  advantage.  It  is  possible  that  by  such  means  a  much 
higher  vacuum  might  be  obtained  than  would  otherwise  be 
possible. 

The  idea  of  a  rubber  bag,  or  diaphragm,  used  in  this  kind  of 
way  is,  however,  not  new.  It  was  described  more  than  ten 
years  ago  in  a  patent  specification  by  Mr.  Steam  in  connec- 
tion with  his  Sprengel  type  of  pump.  Mercury  was  raised  into- 
a  vacuous  chamber  by  the  pressure  of  air  on  the  opposite  side 
of  a  flexible  diaphragm,  so  that  the  air  did  not  come  into- 
actual  contact  with  the  mercury.  The  particular  arrange- 
ment described  is,  however,  somewhat  complicated.  The 
specification  says  :  ' '  Greater  uniformity  and  celerity  of  action 
are  attained  owing  to  the  reduction  of  the  length  of  the 
exhausting  portion  of  the  pump,  and  the  keeping  of  the 
mercury  in  sealed,  exhausted  vessels  during  the  periods  of 
its  circulation."  No  mention,  however,  is  made  of  any  arrange- 
ment for  keeping  out  the  air  between  the  periods  of  the  circula- 


EXHAUSTING.  167 

tion  of  the  mercury — that  is  to  say,  when  the  pump  is  not 
working,  or  when  a  fresh  lot  of  lamps  are  being  attached. 

Stearn  appears  also  to  have  been  the   originator   of  the 
shortened  form  of  pump. 

•  There  is,  however,  a  far  more  serious  trouble  than  that  of 
the  air  film  to  be  guarded  against  in  working  with  mercury 
pumps.  It  is  that  of  moisture.  Any  moisture  in  the  pump 
will  make  a  good  vacuum  an  impossibility.  It  will  be  impos- 
sible to  obtain  an  exhaustion  of  less  than  the  tension  of  the 
water  vapour,  which  is  so  great  that  the  vacuum  obtainable  is 
not  nearly  good  enough  for  a  lamp.  At  the  ordinary  tempera- 
ture of  the  air  the  pressure  is  equal  to  that  of  about  half  an 
inch  of  mercury.  In  order,  therefore,  to  keep  the  pump  dry, 
there  is  always  a  "  drying  tube,"  containing  some  substance 
having  a  great  affinity  for  water,  connected  to  the  pump.  This 
tube  is  usually  inserted  between  the  lamps  and  the  pump,  so 
as  to  prevent  any  moisture  from  the  lamps  from  passing  into 
the  pump.  Calcium  chloride  or  strong  sulphuric  acid  have 
been  employed  as  the  drying  agent,  but  the  only  really  effec- 
tive drier  is  phosphoric  anhydride.  A  tube,  or  tubes,  of  this 
substance  (a  white  powder)  are  connected,  so  that  the  air  pass- 
ing out  of  the  lamps  must  pass  over  it.  Such  a  tube  may  be 
seen  in  Fig.  53,  just  below  the  stand  on  which  the  lamps  are 
resting,  and  also  at  D,  Fig.  55.  The  lamps  should  be  as  dry 
as  possible  before  they  are  sealed  to  the  pump,  or  the  drying 
tubes  will  have  to  be  very  frequently  renewed.  Between  the 
drying  tube  and  the  pump  there  should  be  a  small  bulb  con- 
taining glass  wool,  to  prevent  any  of  the  powder  from  being 
blown  round  into  the  pump. 

Unfortunately,  moisture  can,  and  often  does,  get  into  the 
pumps,  and  seriously  interferes  with  their  working  in  spite  of 
the  drying  tubes.  It  gets  in  along  with  the  air  used  for  rais- 
ing the  mercury.  It  is  naturally  most  troublesome  in  damp 
weather.  It  gets  worked  in  in  exactly  the  same  manner  as  the 
air  film  already  described.  The  trouble  is  greatest  in  pumps 
where  the  mercury  is  raised  by  compressed  air.  The  Author 
has  tried  passing  the  air  over  calcium  chloride  before  it  enters 
the  pump  reservoir.  In  damp  weather  a  lot  of  moisture  was 
absorbed  in  this  way,  but  it  did  not  entirely  prevent  the 
entrance  of  moisture  into  the  pump.  Mr.  Kennedy  appears  to 


168          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

have  adopted  a  similar  arrangement.  In  Fig.  55  the  air  which 
enters  the  reservoir  B  has  first  to  pass  through  the  drying  bulb 
H.  The  method  of  the  rubber  bag,  mentioned  on  p.  166,  might 
be  very  useful  in  keeping  out  moisture.  In  the  case  of  hand 
or  other  pumps,  where  the  mercury  reservoir  is  itself  lifted, 
air  and  moisture  might  be  prevented  from  entering  by  having 
the  top  of  the  reservoir  entirely  closed.  The  reservoir  would 
then  require  raising  30in.  higher  than  it  otherwise  would, 
owing  to  the  absence  of  air-pressure  on  the  mercury. 

The  lamps  should  be  joined  to  the  pump  by  fusion.  Kubber 
and  other  joints  are  not  to  be  relied  on.  In  sealing  a  lamp 
on  to  the  pump  a  hand  blow-pipe  must  be  used,  and  on  no 
account  must  the  lamps  be  blown  into  in  order  to  blow  out 
the  joint,  or  moisture  will  be  introduced. 

When  glass  tubing  is  softened  by  heat  it  always  caves  in- 
wards, and  it  is  usual  in  glass-blowing  to  blow  into  the  tube  in 
order  to  prevent  the  passage  from  being  closed  up.  After  a 
little  practice  it  is  quite  easy  to  seal  on  lamps  without  having 
to  blow  into  them.  If,  however,  it  is  desired  that  the  joints 
be  blown  out,  the  air  contained  in  the  pumps  and  the  lamps 
themselves  can  be  made  to  do  the  work.  When  the  lamps  are 
sufficiently  exhausted  they  are  sealed  off  by  melting  the  narrow 
part  of  the  stem  close  to  the  bulb  by  means  of  the  pointed 
blow-pipe  flame.  The  stem  of  each  lamp  sealed  off  is  there- 
fore left  on  the  pump.  In  sealing  on  fresh  lamps  only  one  of 
these  stems  is  cut  off  at  a  time,  so  that  as  soon  as  the  new  lamp 
is  melted  on  there  is  no  outlet  for  the  air  in  the  pump  and 
the  lamps.  The  heat  of  the  joint  being  made  warms  the  air 
within  the  apparatus,  and  the  air  consequently  exerts  a  pressure 
within,  and  blows  the  softened  glass  at  the  joint  outwards. 
With  a  little  practice  this  can  be  done  with  certainty  to  the 
required  extent  each  time.  If  several  lamps  are  exhausted  at 
the  same  time  on  each  pump,  there  is  a  good  deal  of  time  lost 
in  sealing  on  each  one  separately  in  this  way.  It  is,  there- 
fore, better  to  cut  off  the  whole  of  the  "  fork,"  to  which  the 
lamps  are  attached,  and  seal  on  a  fresh  one  to  which  the 
lamps  have  already  been  attached.  There  is  then  only  one 
joint  to  be  made  on  each  pump,  instead  of  one  for  each  lamp. 

In  order  to  do  the  sealing  on  easily,  it  is  necessary  to  have 
a  small  and  light  blow-pipe.  Fig.  57  shows  one  designed  by 


EXHAUSTING. 


169 


the  Author  for  the  purpose.  It  is  so  small  that  it  can  be  used 
in  places  where  there  is  very  little  room  to  work.  A  is  the 
brass  barrel  about  2in.  long,  divided  by  a  partition,  B,  in  the 
centre.  A  tube,  C,  is  screwed  into  B,  and  forms  the  air  nozzle. 
D  is  the  air-blast  pipe,  and  E  is  the  gas  pipe.  The  air-jet 
tube,  C,  can  be  adjusted  by  screwing  it  in  or  out  with  a  screw- 
driver by  taking  off  the  cap  F.  The  pipes  D  and  E  pass 
through  a  wooden  handle,  G,  and  are  connected  to  the  air  and 
gas  supply.  One  of  these  blow-pipes  may  be  seen  in  Fig.  53. 
The  rubber  air  and  gas  tubes  are  entirely  out  of  the  way,  and 


FIG.  57.— Blow- Pipe  for  Pump-Room. 


do  not  impede  the  manipulation  of  the  blow-pipe  as  they  do 
when  attached  to  the  barrel  in  the  usual  way. 

Testing  the  Vacuum. 

It  is  necessary  in  the  lamp  factory  to  have  some  means  of 
testing  the  degree  of  vacuum  in  the  lamps,  both  while  they 
are  still  on  the  pumps  and  after  they  have  been  sealed  off.  In 
the  early  stages  of  exhaustion  the  height  of  the  mercury 
column  in  the  shaft  or  fall-tube  of  the  pump  will  give  an  indi- 
cation. When,  however,  the  column  has  arrived  at  the  height 
of  the  barometer,  there  are  still  many  degrees  of  exhaustion  to 


170          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

be  passed  through  before  the  vacuum  is  good  enough  for  the 
lamps.  It  is  these  higher  degrees  of  vacuum  that  the  lamp- 
maker  requires  to  test,  and  for  this  purpose  the  height  of  the 
mercury  column  is,  of  course,  no  longer  of  any  use. 

A  gauge  was  invented  some  years  ago  by  McLeod  for  test- 
ing these  higher  degrees  of  exhaustion,  but  it  is  of  little  use  to 
the  lamp-maker.  As,  however,  it  is  the  only  gauge  by  which 
any  estimates  of  actual  pressures  can  be  made,  it  is  here  briefly 
described. 

The  simple  form  of  the  McLeod  gauge  is  represented  in 
Fig.  58.  A  is  a  bulb,  to  the  top  of  which  a  small  closed  gra- 
duated tube,  B,  is  joined  on.  A  side  tube,  C,  also  graduated, 


FIG.  58. — McLeod  Vacuum  Gauge. 

is  joined  on  below  the  bulb.  The  tube  C  communicates  with 
the  vessel  in  which  the  degree  of  vacuum  is  to  be  measured. 
There  is  a  column  of  mercury  in  the  tube  D,  which  terminates 
at  the  lower  end  in  a  movable  reservoir.  When  a  vacuum  is 
to  be  measured,  the  movable  reservoir  is  raised  so  that  the 
mercury  flows  up  D  into  A  and  B  and  the  side  tube  C.  The 
mercury  rises  in  A  in  the  same  way  as  in  the  bulb  of  a  Geissler 
pump,  driving  any  air  contained  in  A  before  it  into  the  tube 
B.  As  the  mercury  rises  this  air  is  more  and  more  com- 
pressed. The  mercury,  also  rising  in  the  open  tube  C,  is  at  a 
higher  level  than  that  in  B,  as  the  air  above  it  is  practically 
unconfined.  The  difference  in  the  height  of  the  columns  C 
and  B  is  a  measure  of  the  pressure  upon  the  air  confined  in  B. 


EXHAUSTING.  171 

The  volume  of  the  air  in  B  is  ascertained  by  the  graduations 
on  the  tube.  The  total  volume  of  the  bulb  A  and  the  tube  B, 
above  the  point  where  the  tube  C  branches  off,  is  also  known. 
If  this  volume  be  represented  by  V,  and  the  volume  of  the  air 
contained  in  this  space  when  compressed  by  the  mercury  into 
B  =  v,  and  the  pressure  indicated  by  the  difference  in  level  of 
the  columns  C  and  B  =  P,  and  if  p  equals  the  pressure  of  the 
vacuous  space  which  it  is  desired  to  know,  then  — 


Measurements  of  vacua  taken  with  this  gauge  have  been  given 
by  various  experimenters,  showing  exhaustions  of  less  than 
one  millionth  of  an  atmosphere.  It  has,  however,  been  pointed 
out  by  Mr.  Swinburne  that  such  results  do  not  take  into  con- 
sideration the  tension  of  the  mercury  vapour,  which  is  stated 
by  some  authorities  to  be  as  much  as  50  millionths  of  an 
atmosphere.  It  is  obvious  that  the  gauge  is  subject  to  any 
errors  arising  from  an  air  film  upon  the  glass.  Mr.  Swinburne 
has  found  that  great  variations  in  the  readings  are  caused  by 
a  slight  heating  of  the  gauge.  Such  variations  might  be 
partly  produced  by  the  air  film  coming  off  the  glass. 

Of  course,  there  must  be  mercury  vapour  within  the  pumps, 
and  it  probably  extends  through  the  often  long  length  (6ft.  or 
8ft.)  of  small  tubing  leading  to  the  lamps.  If  the  pumps  and 
the  lamps  be  at  the  ordinary  temperature  of  a  pump  room, 
say  70°F.,  and  the  lamps  be  sealed  off,  they  will  probably  be 
full  of  mercury  vapour  and  air  at  the  pressure  of  50  millionths 
of  an  atmosphere,  or  whatever  the  pressure  of  mercury  vapour 
really  is  at  that  temperature.  If,  however,  the  mercury  in 
the  pump  is  at  TOdeg.,  and  the  lamps  be  heated  to  near  the 
softening  point  of  the  glass,  and  are  sealed  off  while  at 
that  temperature,  they  will  still  be  full  of  mercury  vapour  and 
air  at  the  same  pressure  as  before  only,  though  at  the  very 
much  higher  temperature.  When  the  lamps  have  cooled,  the 
tension  of  the  mercury  vapour  in  them  will  be  very  much  less 
than  50  millionths  of  an  atmosphere.  It  is,  therefore,  prob- 
able that  lamps  sealed  off  while  hot  will  have  better  vacuums* 
than  if  sealed  off  cold. 

*  In  the  factory  the  plural  "  vacua  "  is  seldom  used. 


172  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

For  a  similar  reason,  it  would  appear  wrong  to  heat  the 
mercury  in  the  pumps.  If  the  mercury  and  pumps  are  heated, 
the  vapour  will  have  a  still  greater  tension  than  50  millionths 
of  an  atmosphere,  and  the  vacuum  will  be  correspondingly 
poorer,  both  as  regards  the  mercury  vapour  and  air.  If  the 
lamps  are  sealed  off  at  a  lower  temperature  than  that  of  the 
mercury,  mercury  vapour  may  even  be  condensed  in  the  lamps. 
In  such  a  case  the  lamps  would  certainly  have  the  full  pres- 
sure of  mercury  vapour  within  them. 

The  behaviour  of  the  McLeod  gauge  is  of  great  interest  and 
importance  with  reference  to  experiments  in  high  vacua ;  but 
how  far  its  indications  may  be  relied  upon  does  not  concern 
the  lamp -maker,  so  far  as  the  ordinary  factory  operations  are 
concerned.  There  are  other  ways,  not  indeed  of  testing  the 
actual  degree  of  vacuum,  but  of  ascertaining  whether  or  not  it 
is  up  to  the  standard  required  for  the  lamp. 

The  behaviour  of  the  pumps  themselves  usually  gives  a  suffi- 
cient indication.  The  amount  of  air  being  taken  out  by  the 
mercury  at  any  time  can  be  easily  seen  in  many  forms  of 
pumps.  In  the  Geissler  form,  when  there  is  a  narrow  tube 
above  the  pump  bulb,  as  in  Figs.  44  and  50,  the  amount  of 
air  coming  out  at  each  stroke  can  be  seen  as  it  passes  through 
this  tube.  When  the  vacuum  is  good,  the  air-bubble  will  be 
very  small.  When  there  is  no  such  tube,  as  in  the  pumps  in 
Fig.  53,  the  small  air-bubble  can  be  seen  as  it  passes  up 
through  the  mercury  in  the  valve-chamber,  as  it  invariably 
passes  up  against  the  glass.  In  the  Sprengel  pump,  when  air- 
bubbles  are  no  longer  to  be  seen  coming  down  the  fall-tubes, 
a  good  vacuum  may  generally  be  relied  upon  after  a  sufficient 
interval  from  the  disappearance  of  the  bubbles.  With  either 
type  of  pump,  provided  that  it  is  in  good  order  and  there  are 
no  leaks  in  the  lamps,  a  good  vacuum  may,  as  a  rule,  be  relied 
upon  after  the  pump  has  been  at  work  for  a  certain  time,  the 
actual  time  depending  on  the  particular  pattern  of  pump  and 
the  number  of  lamps  connected  to  it.  The  time  taken  to  ob- 
tain the  required  degree  of  vacuum  is  not,  however,  directly 
proportional  to  the  number  of  lamps  on  the  pump.  A  pump 
which  will  exhaust  three  lamps  in  one  hour  will  probably  ex- 
haust six  lamps  in  considerably  less  time  than  two  hours.  Up 
to  a  certain  point  the  time  is  approximately  proportional  to 


EXHAUSTING. 

the  number  of  lamps,  but  after  that  point  the  number  of 
lamps  does  not  make  much  difference.  Up  to  the  time  when 
what  might  be  called  the  free  gases  have  been  removed,  the 
time  of  pumping  is  about  proportional  to  the  number  of  lamps 
on  the  pump.  After  that  time,  when  the  air-film  is  being 
reduced  and  the  gases  are  being  driven  out  of  the  filaments, 
the  number  of  lamps  on  the  pumps  does  not  make  much 
difference  to  the  time  required  to  complete  the  exhaustion. 

After  the  lamps  have  been  sealed  up  and  removed  from  the 
pumps,  the  vacuum  is  usually  tested  by  means  of  an  induction 
spark.  An  induction  coil  is  fitted  up  in  a  dark  room  or  closet, 
and  is  arranged  so  that  it  would  give  a  spark  of  about  fin.  in 
length.  The  terminals  are,  however,  set  at  a  greater  distance 
apart  than  this,  so  that  no  spark  passes.  The  lamp  to  be 
tested  is  then  taken  in  the  hand,  and  its  terminals  are  held 
against  one  or  other  of  the  secondary  terminals  of  the  coil. 
If  the  vacuum  is  very  poor,  the  lamp  will  glow  all  through, 
even  before  it  touches  the  coil.  If  it  is  better  than  this,  how- 
ever, but  still  not  good,  there  will  be  a  glow  all  through  the 
lamp  when  its  terminals  touch  the  coil.  If  the  vacuum  is  good 
enough  there  may  still  be  a  considerable  amount  of  glow,  but  it 
will  all  appear  upon  the  inside  surface  of  the  glass,  and  not  at 
all  in  the  interior  of  the  lamp.  The  glow  in  the  interior  of 
the  lamp  in  the  case  of  a  bad  vacuum  will  be  of  a  blue  colour, 
or,  if  the  vacuum  is  very  bad,  of  a  purple  colour.  In  the  case 
of  a  good  vacuum,  when  there  is  a  glow  upon  the  glass  only, 
it  will  be  blue  if  the  bulb  is  made  of  lead  glass,  but  of  a  green 
colour  if  German  glass  is  used.  In  the  case  of  a  good  vacuum 
the  glow  on  the  glass  is  not  continuous,  but  is  intermittent, 
and  only  appears  in  patches ;  or  there  may  be  no  glow  to 
be  seen  at  all.  With  a  good  vacuum,  while  one  hand  holds 
the  lamp  to  one  terminal  of  the  coil,  the  other  hand  may 
grasp  the  other  terminal  with  impunity ;  whereas,  if  the 
vacuum  is  bad,  a  shock  will  be  felt. 

The  coil  test  is,  however,  not  infallible.  A  lamp  which 
shows  a  poor  vacuum  may,  by  being  run  very  bright  for  a 
minute  or  two,  be  made  to  show  a  good  vacuum  on  the  coil, 
no  trace  of  any  glow  being  perceptible.  Advantage  is  taken 
of  this  phenomenon  by  pump-room  operatives,  if  not  properly 
looked  after,  for  getting  poor  vacuums  passed  as  good  ones. 


174          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

The  coil  test  is  sometimes  applied  to  the  lamps,  while  still 
on  the  pumps,  in  order  to  ascertain  if  they  are  ready  for 
sealing  off.  It  is  more  bother  than  it  is  worth,  however,  to  do 
this,  as  the  effect  cannot  properly  be  seen  except  in  the  dark. 

Cleaning   the  Mercury. 

The  mercury  used  in  the  pumps  should  be  as  pure  and  as 
•clean  as  possible.  The  most  effective  way  of  ensuring  this  is 
to  distil  it.  It  may  then  be  freed  from  other  metals,  such  as 
zinc,  lead,  or  tin,  which  it  often  contains.  Mercury  that  is 
free,  or  nearly  so,  from  other  metals  may  be  extremely  dirty, 
-and  absolutely  unfit  for  use  in  the  pumps.  In  this  case  it  can 
usually  be  effectively  cleaned  by  filtering.  A  piece  of  blotting 
paper  may  be  folded  and  put  in  a  glass  funnel,  as  in  filtering 
any  ordinary  liquid.  A  small  pin-hole  must,  however,  be  made 
at  the  bottom.  The  mercury  runs  through  this  hole  in  a  fine 
stream,  and  as  it  descends  in  the  funnel  it  leaves  the  dirt 
which  was  floating  on  its  surface  adhering  to  the  blotting 
jpaper.  If  it  is  very  dirty  it  may  be  advantageous  to  filter  it 
into  a  vessel  containing  strong  sulphuric  acid.  It  must  after- 
wards be  filtered  into  water,  and  again  into  a  dry  vessel.  The 
most  unpromising  looking  mercury  may,  in  this  way,  be  made 
quite  clean. 

Mercury  may  be  distilled  in  an  iron  retort  at  its  normal 
boiling  point.  It  is,  however,  better  to  distil  it  in  a  glass 
apparatus  under  a  vacuum,  so  that  it  distils  over  at  a  much 
lower  temperature,  and  its  progress  can  be  watched.  Various 
forms  of  apparatus  for  distilling  mercury  in  this  way  have 
been  described.  That  shown  in  Fig.  59  will  answer  the  pur- 
pose. A  is  a  flattened  bulb  on  the  end  of  the  tube  B.  The 
lower  end  of  B  reaches  to  near  the  bottom  of  the  open  vessel 
C.  The  tube  E  E  leads  out  of  A,  and  is  carried  down  12in. 
or  so  below  C.  F  is  a  stop-cock  communicating  with  E.  The 
dirty  mercury  is  poured  into  C,  and  as  it  is  distilled  it  collects 
in  G. 

To  start  the  apparatus,  mercury  is  poured  into  C,  and  a  cup 
of  distilled  mercury  is  held  to  the  lower  end  of  E.  The  cock 
F,  connecting  with  a  mechanical  vacuum  pump,  is  opened, 
and  a  vacuum  is  produced  in  A.  The  dirty  mercury,  therefore, 
rises  in  B  and  into  A,  and  the  clean  mercury  rises  to  a  similar 


EXHAUSTING. 


175 


height  in  the  tube  E.  D  is  a  ring  gas  burner  surrounding 
the  tube  B,  just  below  the  vessel  A.  The  gas  is  now  lighted, 
and  heats  the  mercury  in  A.  After  a  while  the  mercury 
in  A  begins  to  distil  over,  the  vapour  condensing  in  the 
tube  E.  When  the  distillation  is  fairly  set  going,  the  cock  F 
may  be  turned  off,  although  it  may  be  necessary  occasionally 
to  open  it  if  the  vacuum  deteriorates  from  any  cause.  With 
a  good  mechanical  pump  the  level  of  the  mercury  in  A  will 
be  nearly  30in.  above  that  in  C,  and  the  level  in  E  a 
little  below  F  will  be  an  equal  height  above  the  bend  in  the 


FIG.  59. — Mercury  Still. 

lower  part  of  E.  The  depth  of  the  vessel  C  is  such  that  if  it 
is  filled  with  mercury  the  mercury  in  A  will  not  rise  high 
enough  to  flow  into  E  and  foul  that  which  has  distilled  over. 
The  level  of  the  mercury  in  A  must  be  watched,  so  that  it 
does  not  fall  too  low.  As  it  distils  over,  more  mercury  must 
be  added  to  C.  If  the  vessel  C  is  a  large  flat  one,  it  will  need 
less  attention  than  if  it  is  a  narrow  one,  as  the  level  of  the 
mercury  in  A  will  be  less  affected  for  a  given  amount  distilled 
over. 

Great  care  must  be  taken  that  the  mercury  poured  into  C  is 
quite  dry.    If  it  is  wet  or  damp,  the  moisture  will  get  into  the 


176          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

tube  B.  The  mercury  in  B  is  hot  for  some  distance  below  A. 
In  the  upper  part  it  is  above  the  boiling  point  of  water.  Any 
moisture  is  gradually  worked  into  the  tube  B,  and  when  it 
reaches  the  hot  part,  it  is  turned  into  steam  and  rises  into  A, 
and  drives  the  hot  mercury  in  A  down  into  B,  so  that  it 
reaches  the  cold  part  of  the  tube,  and  cracks  it  all  to  pieces. 
There  should  be  wire  gauze  between  A  and  the  burner,  so  as 
to  distribute  the  heat.  A  should  also  have  an  asbestos  cover, 
so  that  the  mercury  vapour  is  not  condensed  until  it  enters  E. 

Mechanical  Pumps. 

The  process  of  exhausting  lamps  with  mercury  pumps  is 
costly  and  slow.  Any  mechanical  pump  which  could  quickly 
and  with  certainty  produce  a  sufficiently  good  vacuum  would 
be  welcomed  by  lamp-makers.  The  difficulties  in  the  way  of 
constructing  such  a  pump  are  many.  One  great  difficulty  is 
that  of  preventing  leakage  through  the  glands  and  past  the 
piston.  Another  trouble  is  that  the  working  parts  require 
lubrication.  If  oil  is  used,  there  will  certainly  be  vapours 
given  off,  which  will  prevent  a  good  vacuum  from  being 
obtained.  A  pump  was  exhibited  in  Boston,  U.S.A.,  about 
three  years  ago,  in  which  all  the  difficulties  were  stated  to 
have  been  overcome.  It  was  said  to  exhaust  lamps  in  a  far 
less  number  of  seconds  than  a  mercury  pump  would  do  in 
minutes.  The  chief  point  about  this  pump  was  that  the 
working  cylinder  was  enclosed  in  a  vacuum  jacket,  and  in 
this  way  all  leakage  into  the  working  cylinder  was  prevented. 
Oil  was,  however,  used  in  the  pump.  As  nothing  appears  to 
have  been  published  regarding  this  pump  since  it  was  first 
exhibited,  it  is  to  be  presumed  that  its  performance  did  not 
after  all  come  up  to  the  expectations  of  the  inventors. 
.  An  ingenious  mechanical  pump,  containing  mercury  in  the 
cylinder,  was  patented  in  1882  by  Gimingham,  but  the  Author 
does  not  know  to  what  extent  it  was  found  to  answer.  A 
hollow  plunger,  open  at  the  bottom,  is  caused  to  work  up 
and  down  in  a  cylinder,  which  is  about  two-thirds  full  of 
mercury.  A  tube  passes  through  the  centre  of  the  bottom 
of  the  cylinder  to  a  point  about  three  parts  up  the  cylinder. 
The  lower  end  of  this  tube  is  connected  to  the  vessel  to  be 
exhausted,  while  a  valve  rests  upon  its  upper  end  within 


EXHAUSTING.  177 

the  cylinder.  There  is  also  a  valve  in  the  upper  part  of  the 
plunger,  the  two  valves  being  connected  by  a  string.  When 
the  plunger  is  down,  it  is  entirely  filled  with  mercury.  On 
its  being  raised,  a  vacuum  is  produced  above  the  mercury, 
and  air  from  the  vessel  to  be  exhausted  is  drawn  in  through 
the  inner  tube,  the  valve  being  automatically  raised  by  the 
string.  On  the  plunger  being  again  depressed  the  lower 
valve  is  closed  by  means  of  a  spring,  and  the  entrapped  air 
is  forced  through  the  plunger  valve,  which  remains  sealed 
by  some  of  the  mercury  which  passes  through  behind  the  air. 

A  rotary  mercurial  air-pump  was  described  by  Dr.  F. 
Schulze-Berge  in  a  Paper  read  before  the  International 
Electrical  Congress  at  Chicago,  1893.  This  class  of  pump 
has  to  contend  with  the  troubles  of  sliding  connections, 
which  must  be  formed  in  such  a  way  as  to  be  absolutely 
air-tight ;  the  alternative  being  that  the  vessel  which  is 
required  to  be  exhausted  must  rotate  with  the  pump.  Good 
results,  however,  appear  to  have  been  obtained  with  a  sliding 
connection. 

Pumps  for  producing  the  vacua  for  working  the  mercury 
pumps  and  for  flashing,  &c.,  do  not  require  to  be  of  any  very 
special  construction.  The  valves  must,  however,  be  opened 
and  closed  by  direct  mechanical  action,  and  should  not  depend 
on  the  pressure  of  the  air.  Silk  valves  will  not  answer,  as 
they  wear  out  much  too  quickly,  and  are  not  suitable  for 
large  pumps. 


CHAPTER  XIII. 


TESTING. 

THE  lamps,  having  been  exhausted,  are  next  tested  for  volt- 
age and  candle-power,  and  for  this  purpose  it  is  necessary  to 
have  a  photometer  and  instruments  for  measuring  the  current 
and  the  pressure.  The  object  of  the  test  is  to  find  at  what 
voltage  the  lamps  must  be  run  in  order  that  they  shall  be  at 
the  right  temperature — the  temperature,  that  is,  at  which 
they  are  required  to  be  run,  3,  3J,  4,  or  other  number  of 
watts  per  candle-power  as  desired. 

In  some  factories  only  a  few  lamps  are  tested  on  the  photo- 
meter, and  the  other  lamps  are  compared  directly  by  eye  with 
these.  Thus,  if  100-volt  lamps  are  being  made,  lamps  will 
be  selected  by  the  photometer  which  run  at  the  required  tem- 
perature at,  say,  94,  96,  98,  100,  102,  104,  and  106  volts. 
These  seven  lamps  are  all  hung  up  in  a  line.  The  lamps  to 
be  tested  are  lighted  up  on  the  same  circuit,  and  in  close 
proximity  to  the  standards.  They  are  then  labelled  accord- 
ing as  they  are  found  to  compare  with  the  standards. 

This  is  not,  however,  a  satisfactory  method  of  testing.  The 
eyes  of  the  tester  soon  become  unreliable,  and  refuse  to  see 
considerable  differences  in  the  temperature  of  the  filaments. 
Darkened  glasses  may  be  used  to  save  the  eyes  from  the  in- 
jurious effects  of  continually  looking  at  the  bright  filaments, 
but  they  do  not  make  the  testing  any  easier. 

Far  greater  accuracy  can  be  obtained  by  testing  each  lamp 
separately  on  the  photometer.  Those  accustomed  to  make 
photometric  tests  of  lamps  in  the  laboratory  will,  perhaps, 
think  that  it  would  be  impracticable  to  test  every  individual 
lamp  owing  to  the  time  occupied  in  making  the  test.  A 

N  2 


180          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

photometer  can,  however,  be  arranged  for  the  lamp  factory  so 
that  the  testing  can  be  done  very  quickly.  A  thousand  lamps 
a  day  can  easily  be  tested  on  one  photometer,  provided  that 
the  lamps  do  not  differ  greatly  from  each  other ;  that  is  to 
say,  that  100  volt  and  70  volt,  or  8-c.p.  and  20-c.p.  lamps  are 
not  all  mixed  up  together.  In  the  laboratory  the  testing  ap- 
paratus is  conveniently  arranged  for  the  observer  to  be  able  to 
read  the  photometer  and  the  different  instruments  without 
moving  from  his  seat ;  but  in  the  factory,  for  rapid  testing, 
the  apparatus  is  arranged  so  as  to  be  operated  by  two  or 
three  persons.  A  wattmeter  is  also  required  in  addition  to  the 
other  instruments.  A  wattmeter  is  not  a  necessity  in  the  labora- 
tory, as  the  calculations  can  be  done  on  a  slide  rule  with  suf- 
ficient rapidity  ;  but  it  is  essential  for  factory  testing,  as  it  does 
away  with  the  necessity  for  making  any  calculations  whatever. 
The  great  trouble  in  all  photometric  measurements  is  the 
want  of  a  really  good  standard  of  light.  The  light  of  the 
incandescent  lamp  is  estimated  in  candles.  Standard  candles 
are,  however,  of  absolutely  no  use  for  factory  lamp  testing, 
and  are  of  little  value  as  standards  under  any  circumstances. 
The  most  reliable  standard  of  light  is  the  Harcourt  pentane 
lamp  (second  pattern).  It  is  simply  a  lamp  burning  the 
vapour  of  standard  quality  pentane.  It  is  arranged  so  that 
the  same  size  of  flame,  giving  a  light  of  one  or  two  average 
standard  candles,  is  always  presented  to  the  photometer.  It 
is,  however,  more  of  a  laboratory  standard  than  one  fitted  for 
constant  use  with  the  factory  photometer. 

For  actual  use  in  the  factory  an  Argand  burner,  burning 
ordinary  illuminating  gas  with  a  Methven  screen,  is  prefer- 
able, as  it  requires  less  attention.  The  chief  trouble  in  using 
an  Argand  burner,  even  with  a  Methven  screen,  is  due  to  the 
flickering  to  which  the  flame  is  liable  if  there  is  the  slightest 
draught.  This  can,  however,  be  remedied  by  enclosing  the 
burner  in  a  large  box  having  openings  sufficient  only  for  the 
proper  supply  of  air  to  the  flame.  The  Methven  screen,  it  is 
well  known,  is  simply  a  metal  screen  with  a  hole  in  it  of  such 
a  size  as  to  allow  only  a  small  piece  of  the  centre  of  the  flame 
to  throw  any  light  into  the  photometer,  the  light  from  this 
piece  of  the  flame  being  very  much  more  constant  than  is  that 
of  the  whole  flame. 


TESTING.  181 

The  position  of  the  Methven  Argand  standard  is  adjusted 
by  comparison  with  the  Harcourt  standard,  and  can  be  checked 
by  it  as  often  as  is  thought  necessary.  If  the  lamp  factory 
has  a  separate  laboratory,  with  a  photometer  and  testing 
instruments,  it  is  more  convenient  to  test  a  lamp  in  the  labo- 
ratory, using  the  Harcourt  standard,  and  then  to  give  the 
same  lamp  to  the  testers  in  the  factory  testing  room,  and  see 
if  their  test  of  it  agrees  with  the  laboratory  one.  A  conve- 
nient way  of  keeping  a  check  on  the  testing  is  to  keep  one  or 
two  lamps  to  be  always  tested  first  whenever  the  photometer 
is  used.  As  long  as  these  standardised  lamps  always  test  the 
same  it  may  safely  be  assumed  that  the  whole  apparatus  is  in 
good  order. 

As  regards  the  photometer  itself,  the  Bunsen  type  is  the 
most  convenient.  The  ordinary  grease-spot  may  be  used,  or 
what  is  known  as  the  Leeson  disc.  The  former  answers  very 
well,  but  the  latter  is  preferred  by  some  testers.  The  Leeson 
disc  is  composed  of  a  piece  of  cardboard,  having  a  star-shaped 
hole  cut  out  of  it,  and  is  covered  with  a  thin  sheet  of  paper  on 
each  side.  Whichever  disc  is  used,  it  is  placed  in  a  small  box 
so  as  to  be  in  a  direct  line  between  the  two  lights,  to  be  com- 
pared with  its  plane  at  right  angles  to  such  line.  One  side  is 
therefore  illuminated  by  the  standard  light,  and  the  other  by 
the  lamp  under  test.  A  mirror  is  fixed  on  each  side  of  the  disc 
at  an  angle  so  that  the  observer  sitting  in  a  direct  line  with 
the  plane  of  the  disc  can  simultaneously  see  both  sides  of  it 
reflected  in  the  mirrors.  When  one  side  of  the  disc  is  more 
powerfully  illuminated  than  the  other,  the  grease  spot, 
or  the  star,  whichever  is  used,  will  appear  darker  on  that  side 
than  the  rest  of  the  disc,  while  on  the  side  less  illuminated  it 
will  appear  brighter.  When  both  sides  are  equally  illuminated, 
the  grease  spot,  if  the  disc  be  a  good  one,  will  be  almost  en- 
tirely invisible.  When  comparing  lights  of  different  colour, 
it  will  never  disappear  altogether,  but  a  correct  balance  can, 
nevertheless,  be  obtained  by  increasing  and  diminishing  the 
intensity  of  the  illumination  on  one  side  of  the  disc  to  an  ex- 
tent which  is  seen  to  be  definitely  above  and  below  the  balance, 
and  gradually  obtaining  a  mean  between  the  two  readings. 

The  disc  should  be  mounted  in  such  a  way  that  it  can  be 
easily  reversed.  The  side  which  is  presented  to  the  gas  light 


182          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

after  a  while  sometimes  turns  yellow,  and  ceases  to  give  cor- 
rect indications ;  a  check  should,  therefore,  be  kept  upon  it 
by  reversing  its  position  and  ascertaining  if  it  gives  the  same 
reading  each  way.  A  new  disc  should  also  be  carefully  tested. 

In  testing  gas  or  other  lights  produced  by  combustion  it  is 
necessary  that  both  the  lights,  the  standard  and  the  one  under 
test,  be  fixed.  The  photometer  disc  must,  therefore,  be  mov- 
able between  the  one  and  the  other  and,  as  a  consequence,  the 
observer  has  to  stand  up  and  move  with  the  disc.  Fortunately 
for  those  who  have  to  test  incandescent  lamps,  matters  can  be 
so  arranged  that  it  is  not  necessary  for  them  to  stand  up  and 
crane  their  necks  all  day.  The  incandescent  lamp  may  be 
moved  and  the  photometer  disc  may  be  fixed.  The  observer 
can,  therefore,  sit  comfortably  in  a  chair  while  testing. 

It  is  not  intended  here  to  go  into  the  matter  of  photometry 
and  light  standards  generally,  as  there  are  already  works  upon 
the  subject.  An  excellent  one  is  that  by  Mr.  W.  J.  Dibdin, 
entitled  "  Practical  Photometry,"  which,  although  it  deals 
chiefly  with  gas-testing,  contains  much  information,  useful 
to  the  incandescent  electric  lamp  tester. 

In  testing  incandescent  lamps  the  object,  as  already  men- 
tioned, is  to  find  what  difference  of  potential  must  be  applied 
at  the  terminals  of  the  lamp  in  order  that  it  may  be  main- 
tained at  a  definite  temperature  or  efficiency  in  watts  per 
candle-power.  It  is  therefore  necessary  to  light  up  the  lamp 
and  ascertain  the  amount  of  power  it  is  taking,  and  then  see 
if  it  is  giving  the  required  amount  of  light  for  that  amount 
of  power.  If  the  lamps  are  to  be  run  at  4  watts  per  candle- 
power,  then  for  every  4  watts  supplied  to  the  lamp  it  should 
give  a  light  of  one  candle.  If  the  lamp  when  lighted  up  is 
found  not  to  give  as  much  light  as  one  candle  for  every  four 
watts,  or  is  found  to  give  more  than  that  amount,  then  it  must 
be  run  brighter  or  duller  by  applying  a  greater  or  less  differ- 
ence of  potential  until  it  attains  the  required  degree  of  bril- 
liancy. When  the  adjustment  is  found  to  be  correct  the 
voltmeter  indication  is  read,  and  the  number  of  volts  registered 
is  the  proper  voltage  for  that  lamp.. 

This  system  of  testing,  which  was  introduced  by  Mr.  Swin- 
burne, is  the  only  correct  one.  It  is  obvious  that  if  the  lamps- 
are  tested  at  a  fixed  voltage  or  candle-power  they  will  vary  in 


TESTING.  183 

brightness  and  efficiency  (watts  per  candle-power)  unless  they 
are  exactly  alike  hi  every  particular,  whereas  by  this  method 
they  are  tested  at  a  fixed  brightness  and  efficiency,  and  the 
voltage  and  candle-power  are  assigned  in  accordance  there- 
with. 

The  way  in  which  the  Author  recommends  the  use  of  this 
method  in  the  lamp  factory  will  now  be  described. 

The  photometer  table  is  at  such  a  height  above  the  floor 
that  the  lamp  and  the  photometer  disc  are  on  a  level  with  the 
eyes  of  the  observer  when  sitting  comfortably  in  a  chair.  The 
gas  lamp  with  its  Methven  screen  and  the  disc  box  are  at  the 
right  hand  end  of  the  table,  the  lamp  under  test  being  on  the 
left  hand  of  the  observer.  The  photometer  bar  is  graduated 
directly  hi  candles,  the  zero  point  being  immediately  below 
the  disc.  The  candle-power  graduations  are  proportional  to 
the  square  of  their  distance  from  the  zero  line. 

The  lamp  is  carried  in  a  frame  which  can  be  moved  to  and 
fro  along  the  photometer  bar  by  the  observer  at  the  disc.  The 
lamp  holder  is  also  capable  of  being  raised  or  lowered  and 
rotated.  There  is  also  fixed  to  this  frame  another  bar,  also 
divided  into  candles,  but  having  its  zero  mark  at  the  lamp 
end.  This  bar  slides  backwards  and  forwards  with  the  lamp 
frame,  and  the  observer  at  the  photometer  is  therefore  able  to 
read  the  candle-power  of  the  lamp  by  the  indication  on  this 
bar  at  the  zero  line  immediately  below  the  disc,  and  does  not 
have  to  look  along  the  bar  to  where  the  lamp  happens  to  be 
in  order  to  get  the  reading.  This  is  a  great  convenience,  as 
the  observer  does  not  get  the  light  of  the  lamp  into  his  eyes, 
as  is  the  case  in  the  former  method. 

The  wattmeter  is  fixed  on  a  shelf  just  behind  and  above 
the  disc  box,  so  that  its  indications  can  be  readily  seen  by  the 
observer  at  the  disc,  the  scale  of  the  instrument  being  an  up- 
right one,  and  not  one  which  has  to  be  looked  down  upon. 
The  wattmeter  scale  is  preferably  divided  directly  in  candles 
at  four  or  other  number  of  watts  to  the  candle  ;  or  there  may 
be  several  scales,  one  above  the  other,  for  testing  at  2,  2J,  3, 
3J,  4,  &c.,  watts  per  candle. 

There  is  a  large  screen  all  round  the  disc  box,  so  that  the 
observer  at  the  disc  does  not  see  any  light  except  that  upon 
the  disc.  No  light,  either  from  the  lamp  under  test  or  the 


184          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE 

gas  lamp,  can  fall  upon  his  eyes.  The  wattmeter  scale  is 
illuminated  sufficiently  by  a  small  incandescent  lamp  run 
very  dull.  The  scale  of  candles  on  the  movable  photometer 
bar,  being  large  and  on  a  white  ground,  can  be  read  by  the 
small  amount  of  diffused  light  from  the  lamp  under  test.  In 
this  way  there  is  no  light  which  can  upset  the  eye  of  the 
observer. 

The  switch  and  variable  resistance  for  adjusting  the  current, 
as  well  as  the  handle  for  moving  the  lamp  to  and  fro,  are  fixed 
in  a  convenient  situation  for  being  worked  by  the  observer. 
The  voltmeter  (and  ammeter,  if  used)  is  situated  on  the  other 
side  of  the  screen  surrounding  the  observer,  and  is  read  by 
one  of  the  assistants.  The  method  of  working  is  as  follows  : 

The  tester-in-chief  sits  at  the  photometer.  In  this  position 
he  can  see  the  disc  box,  the  wattmeter  scale  above  it,  and 
the  candle-power  scale  below  it.  He  does  not  see  either  the 
lamp  under  test  or  the  standard  gas  burner,  nor  the  voltmeter 
or  ammeter,  but  he  has  the  control  of  current  and  of  the 
motion  of  the  frame  containing  the  lamp  under  test.  There 
are  two  assistants  on  the  left  hand  of  the  tester,  one  on  each 
side  of  the  photometer  table.  Before  being  brought  into  the 
photometer  room  the  lamps  have  each  had  a  small  label 
stuck  upon  the  neck. 

Assistant  No.  1  now  puts  a  lamp  into  the  clip  or  holder  in 
the  movable  carrier.  If  the  lamps  are  supposed  to  be  16-c.p. 
ones  the  tester  sets  the  lamp  at  the  distance  for  16  c.p.  by  the 
scale  in  front  of  him.  He  then  turns  the  current  on,  and 
brings  it  up  until  there  is  a  balance  upon  the  screen.  He 
then  looks  up  at  the  wattmeter  scale  to  see  how  many  candles 
the  lamp  should  be  giving  for  the  amount  of  power  it  is  taking. 
If  the  wattmeter  indicates  16  candles,  he  tells  assistant  No.  1 
to  read  the  voltmeter,  and,  whatever  the  number  of  volts 
happens  to  be,  that  is  the  voltage  of  the  lamp. 

Supposing,  however,  when  the  lamp  is  giving  16  candles 
that  the  wattmeter  indicates,  say,  a  little  over  17  candles. 
The  lamp  in  such  a  case  must  then  be  run  brighter  until  its 
actual  candle-power  agrees  with  the  indication  of  the  watt- 
meter. As  the  lamp  is  made  brighter,  there  is,  of  course, 
more  power  spent  in  the  filament,  and  consequently  the 
wattmeter  indication  also  rises,  but  as  the  light  increases 


TESTING.  185 

much  faster  than  the  power,  the  actual  amount  of  light  will 
agree  with  the  wattmeter  indication  at  about  18  candles. 
The  balance  being  obtained,  the  voltmeter  is  read  by  assistant 
No.  1.  The  current  is  turned  off  the  lamp,  and  assistant 
No.  2  takes  it  out  of  the  holder  and  writes  the  voltage  on  the 
label,  while  assistant  No.  1  puts  the  next  lamp  into  position. 
If  the  candle-power  is  also  to  be  marked  on  the  label  the 
assistant  can  ascertain  its  value  for  himself  by  noting  the 
position  of  the  lamp  carrier  on  the  fixed  candle-power  scale. 

As  most  electrical  measuring  instruments  take  a  perceptible 
amount  of  time  to  come  to  rest  after  the  current  is  turned  on, 
it  is  necessary  for  rapid  testing  that  the  instruments  be  not 
allowed  to  return  to  zero  between  the  testing  of  each  lamp. 
For  this  reason,  instead  of  turning  the  current  off  the  whole 
apparatus  it  is  simply  switched  off  the  lamp  just  tested  to 
another  lamp  of  similar  size  located  in  a  convenient  place  for 
giving  light  to  the  assistant  testers  for  writing  the  labels  and 
fixing  the  new  lamp  in  position.  The  next  lamp  being  in 
position,the  current  is  switched  back  again,  and  the  instruments 
quickly  arrive  at  their  readings,  and  do  not  oscillate  as  they 
would  if  they  had  started  from  zero.  The  instruments  must, 
of  course,  be  so  constructed  that  they  may  be  left  continuously 
in  circuit  without  their  accuracy  being  impaired  by  heating. 

The  photometer  room  is  necessarily  quite  dark,  and  painted 
a  dead  black  so  as  not  to  reflect  light.  It  should,  however, 
be  of  ample  size.  It  is  absurd  to  locate  the  testing  apparatus 
in  a  room  in  which  there  is  hardly  space  to  turn  round,  as  is 
frequently  done.  It  is  also  advisable  to  arrange  for  proper 
ventilation.  It  is  not  necessary,  in  order  to  keep  out  day- 
light, that  the  room  should  be  hermetically  sealed  after  the 
manner  of  the  ordinary  photographer's  dark  room. 

The  tester  should  be  in  the  dark  room  for  at  least  ten 
minutes  before  testing,  in  order  that  his  eyes  may  get  accus- 
tomed to  the  subdued  light.  On  no  account  should  he  look  at 
the  lamp  while  testing,  or  his  eye  will  be  unreliable  for  some 
time  afterwards.  There  may,  however,  be  a  piece  of  dark 
neutral-tinted  glass  in  the  screen  on  either  side,  so  that  he 
can  see  the  lamp  and  the  flame  of  the  Argand  burner  without 
any  danger  of  his  eyes  being  effected.  The  Argand  burner 
must  be  lighted  ten  minutes  or  more  before  it  is  required,  as 


186          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

it  may  not  give  the  proper  amount  of  light  until  the  chimney 
and  other  surroundings  are  at  the  normal  working  tem- 
perature. 

Great  care  should  be  taken  with  the  measuring  instruments 
to  see  that  they  are  correctly  calibrated  and  can  be  relied 
upon  all  over  their  scales. 

In  measuring  the  power  taken  by  a  lamp  with  an  ammeter 
and  voltmeter  certain  precautions  are  necessary.  If  the  volt- 
meter is  connected  directly  to  the  lamp  terminals,  so  as  to- 
indicate  the  volts  on  the  lamp,  the  ammeter  must  of  necessity 
be  connected  so  as  to  measure  the  current  through  the  lamp 
plus  that  through  the  voltmeter.  On  the  other  hand,  if  the 
ammeter  is  connected  so  as  to  measure  the  current  of  the 
lamp  only,  the  voltmeter  must  be  connected  to  measure  the 
difference  of  potential  over  the  lamp  plus  the  ammeter.  In 
the  former  case  there  will  be  an  appreciable  error,  unless  the 
voltmeter  has  a  very  high  resistance  compared  with  that  of 
the  lamp  ;  and  in  the  latter  case  there  will  be  an  error  unless 
the  resistance  of  the  ammeter  is  so  small  as  to  be  negligible 
in  comparison  with  that  of  the  lamp.  Instruments  can  be 
made  of  such  resistances  that  in  either  method  of  connecting 
the  error  is  negligible. 

A  wattmeter  may  be  regarded  as  a  combination  of  a  volt- 
meter and  an  ammeter,  and  is  therefore  liable  to  error  from 
the  same  causes.  The  resistances  of  the  wattmeter  cannot, 
however,  be  well  made  of  such  amounts  that  the  error  is 
negligible,  and  it  therefore  becomes  necessary  to  compensate 
for  the  error.  This  may  be  done  by  means  of  a  double  winding 
on  the  current  coil,  each  winding  having  the  same  number  of 
turns.  The  pressure  coil  of  the  wattmeter  is  connected 
directly  to  the  terminals  of  the  lamp.  The  lamp  current,  plus 
the  current  through  the  pressure  coil,  therefore,  passes  through 
the  main  winding  on  the  current  coil  of  the  wattmeter.  The 
current  through  the  pressure  coil  alone  is,  however,  taken 
through  the  supplementary  winding  on  the  current  coil  in  the 
reverse  direction.  The  current  of  the  pressure  coil  thus 
traverses  the  current  coil  an  equal  number  of  times  in  oppo- 
site directions,  and  its  effect  is  therefore  neutralised,  and  the 
instrument  gives  a  correct  reading  of  the  power  spent  in  the 
lamp.  In  order  that  the  wattmeter  shall  have  as  wide  a  range 


TESTING. 


187 


as  possible,  the  current  coil  may  be  again  wound  in  two  equal 
windings,  which  can,  by  means  of  a  switch,  be  connected  up 
either  in  series  or  parallel.  This  arrangement  necessitates 
the  compensating  winding  being  wound  in  two  sections  in 
the  same  way.  There  are,  therefore,  really  four  windings  on 
the  current  coil. 

Fig.  60  is  a  diagram  of  the  connections.  P  is  the  watt 
meter  pressure  coil,  C  C  the  main  current  coil,  and  K  K  the 
compensating  current  coil.  The  two  parts  of  these  coils 
can  be  respectively  connected  in  series  or  parallel  by  the 
switches  S3  and  S4.  L  is  the  lamp  under  test,  and  I  is  the 
subsidiary  lamp,  which  is  switched  on  by  the  switch  S2  for  the 

WATTMETER 


FIG.  60. — Electrical  Connections  for  Testing  Lamps  on  Photometer. 

purpose  already  explained  when  the  lamp  L  is  being  changed. 
R  is  the  variable  resistance,  and  SL  is  the  main  switch.  V  is 
the  voltmeter,  and  A  the  ammeter.  It  will  be  noticed  that 
the  voltmeter  current,  as  well  as  that  through  the  pressure 
coil  of  the  wattmeter,  passes  through  both  the  windings  of 
the  current  coil  of  the  wattmeter,  and  its  effect  is  therefore 
neutralised. 

If  an  ammeter  is  used  it  may  be  placed  where  it  is  shown 
in  the  diagram,  provided  that  it  has  a  resistance  extremely 
small  compared  with  that  of  the  lamp  under  test.  If,  how- 
ever, its  resistance  is  not  very  small,  it  must  be  connected  so 
as  to  take  the  whole  of  the  current,  including  that  of  the 
voltmeter  and  the  wattmeter  pressure  coil.  In  this  case  it 
should  have  a  double  winding,  precisely  as  the  current  coil  of 


188          THE    INCANDESCENT    LAMP   AND    ITS   MANUFACTURE. 


the  wattmeter,  so  that  it  indicates  only  the  current  through 
the  lamp.     The  resistance  of  the  coils  K  K  is  negligible. 


FIG.  61. — Evershed's  Wattmeter  for  Lamp  Testing.     Sectional  Elevation. 
(For  reference  letters  see  Fig.  63.) 

In  a  wattmeter  constructed  by  the  Author  on  the  above 
lines  most  of  the  high  resistance  wire  was  wound  on  the  sus- 


FIG.  62. — Everehed's  Wattmeter  for  Lamp  Testing.     Sectional  Plan. 
(For  reference  letters  nee  Fig.  63.) 

pended   coil,  which   was   consequently  heavy,  and   required 
a  vane  moving  in  glycerine  in  order  to  damp  its  oscillations. 


TESTING. 


189 


Mr.  Evershed  has  designed  a  wattmeter  for  lamp  testing  of  a 
more  delicate  construction,  the  suspended  coil  being  very  much 
lighter  than  in  the  instrument  the  Author  has  used.  By  the 
courtesy  of  Mr.  Evershed  the  Author  is  able  to  give  the  ac- 
companying sketches  of  the  instrument  (Figs.  61  and  62). 
Fig.  63  is  a  diagram  of  the  connections,  which  are  similar  to 
those  in  Fig.  60.  There  are  two  small  pressure  coils  P  P, 
one  on  each  side  of  an  aluminium  vane  D.  This  vane  fits  the 
vane  box  B  only  very  loosely,  but  it  is  so  large  and  light  that 
the  air  damps  out  the  oscillations  in  about  two  seconds.  The 


FIG.  63.—  Diagram  of  Connections  of  Evershed's  Wattmeter. 

H — ,  Main  Terminals  ;  T,  Terminal  of  "  Pressure  "  Coil  Circuit ;  t,  Terminal  for 
Voltmeter ;  M,  Main  "  Current "  Coils  ;  C,  Compensating  Coil  for  Pressure  Coil  and 
Voltmeter  Current ;  P,  Pressure  Coils,  in  Series  with  Platinoid  Resistance  B  ;  DB, 
Vane  Box  for  Damping  Oscillations  ;  D,  Aluminium  Vane,  to  which  P  P  is  fixed  ; 
F,  Bifllar  Suspension,  serves  to  lead  Current  into  PP  ;  A,  Plate  carrying  Pulley  and 
capable  of  Rotation  to  adjust  Zero  ;  I,  Index  ;  S  P,  Switch  to  couple  AI  M  and  C  C  in 
Series  or  in  Parallel  and  obtain  double  range  ;  V,  Voltmeter  for  Measuring  Pressure 
on  Lamp  Terminals  ;  L,  Lamp  under  Test. 

bifilar  suspension  F  passes  over  a  pulley  at  the  top,  and  hi 
this  way  it  is  ensured  that  the  strain  is  equally  divided.  The 
suspension  is  silk  where  it  passes  over  the  pulley,  but  just 
below  the  pulley  it  is  wire,  and  serves  to  convey  the  current 
to  the  coils  P  P.  Fine  wires  lead  the  current  to  and  from  the 
suspension  wires  at  their  junction  with  the  silk.  In  series 
with  the  pressure  coils  P  P  is  a  platinoid  resistance,  B, 
situate  in  the  base  of  the  instrument.  M  is  the  main  current 
coil,  and  C  is  the  compensating  coil  for  the  current  through 
the  pressure  coil  and  the  voltmeter.  T  is  the  terminal  of  the 
pressure  coil  circuit,  t  is  the  terminal  for  the  voltmeter.  The 


190          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

main  terminals  are  marked  +  and  -  .  A  is  a  plate  carrying 
the  pulley,  and  is  capable  of  rotation  for  adjusting  the  zero. 
S  P  is  a  switch  for  coupling  the  coils  M  M  and  C  C  respec- 
tively in  series  or  in  parallel,  and  so  obtaining  a  double  range. 
V  is  the  voltmeter,  and  L  is  the  lamp  under  test.  I  is  the 
index  of  the  instrument.  This  arrangement  of  the  index  and 
scale  is  not  good  for  rapid  testing  in  the  manner  already 
described,  as  the  observer  at  the  photometer  has  to  turn  away 
from  the  photometer  in  order  to  read  the  instrument,  which 
must  be  placed  on  one  side  so  that  he  can  look  down  upon  it. 

The  method  of  compensating  the  wattmeter  and  ammeter 
for  the  error  due  to  the  voltmeter  current  was  devised  by 
Mr.  Swinburne. 

For  laboratory  testing,  where  speed  is  not  so  important,  it 
is  sufficient  to  test  with  an  ammeter  and  voltmeter  only.  The 
ammeter,  if  of  an  appreciable  resistance,  should  be  connected 
so  as  to  measure  the  current  through  the  lamp  plus  the  volt- 
meter current.  A  table  of  the  voltmeter  current  for  each  volt 
can  be  hung  up  in  a  convenient  place,  and  the  amount  can  be 
deducted  from  the  ammeter  reading  at  once. 

The  term  candle-power  of  an  incandescent  lamp  is  generally 
understood  to  denote  the  mean  horizontal  candle-power,  and 
not  the  mean  spherical  candle-power. 

If  the  filament  of  the  lamp  is  circular,  a  single  measurement 
on  the  photometer  is  enough  to  obtain  a  reading  sufficiently 
close  to  the  mean  horizontal  candle-power,  provided  that  the 
measurement  is  not  taken  in  the  plane  of  the  loop,  and  that 
the  two  sides  of  the  filament  are  at  a  considerable  distance 
apart  compared  with  their  diameter. 

If  the  filament  is  not  circular,  the  mean  horizontal  candle- 
power  can  be  obtained  by  taking  the  mean  of  a  number  of 
measurements  all  round  the  lamp,  or  the  lamp  may  be  rapidly 
rotated  while  the  test  is  being  made. 

The  mean  horizontal  candle-power  is  the  candle-power  which 
would  be  given  by  a  straight  circular  filament  of  the  same 
length  and  circumference. 

In  the  case  of  rectangular  filaments,  of  which  the  width  and 
thickness  are  known,  the  mean  horizontal  candle-power  can 
be  ascertained  by  taking  the  measurement  of  the  candle-power 


TE.STING.  191 

with  the  flat  side  of  the  filament  towards  the  photometer,  and 
multiplying  by  a  constant. 
If  u>  =  width  of  the  filament  and  t  =  thickness  of  the  filament, 

the  value  of  the  constant  will  be  0-64 (l  +  -\     If  w  =  1-77  t, 

\       «•/ 

then  the  flat  measurement  is  also  the  mean. 

If  many  lamps  with  the  same  ratio  of  width  to  thickness 
have  to  be  tested,  a  more  convenient  plan  is  to  move  the 
standard  burner  closer  to  or  further  from  the  photometer  disc, 
so  that,  when  measuring  the  lamp  flatways,  the  mean  candle- 
power  is  read  directly  on  the  photometer  scale. 

If  the  ratio  of  width  to  thickness  be  not  known,  a  measure- 
ment may  be  taken  with  the  broad  side  of  the  filament  towards 
the  photometer,  and  another  at  right  angles  to  it  with  the 
narrow  side  towards  the  photometer.  If  the  candle-power  in 
the  former  position  =  W  and  in  the  latter  =  T,  then  the  mean 
candle-power  will  be  0*64  (W  +  T),  provided  that  one  side  of 
the  filament  does  not  screen  the  other  in  making  the  test. 

The  direction  of  the  minimum  light  from  a  flat  filament  is 
at  right  angles  to  the  thin  edge.  The  direction  of  the  maxi- 
mum light,  however,  is  not  that  at  right  angles  to  the  flat 
side,  but  at  right  angles  to  a  line  taken  diagonally  across  the 
filament  from  opposite  corners. 

In  testing  flat  filament  lamps  it  is  possible,  by  turning 
the  lamp,  to  find  the  position  of  maximum  and  minimum 
light.  If  the  ratio  of  the  width  to  the  thickness  be  not  known, 
the  mean  candle-power  can,  nevertheless,  be  found  by  a  single 
measurement  on  the  photometer.  The  lamp  under  test  must 
be  rotated  until  it  is  seen,  by  looking  at  the  photometer  disc, 
that  it  is  in  the  position  in  which  it  gives  the  maximum  read- 
ing. Let  this  maximum  candle-power  =  H.  The  lamp  is  then 
turned  through  an  angle  j8,  until  it  is  seen  that  it  is  in  the 
position  of  minimum  candle-power.  The  mean  candle-power 
is  then  equal  to 

0-64  H  (sin  /?  +  cos  ft). 

The  most  accurate  method  of  obtaining  the  mean  horizontal 
candle-power  is  probably  that  of  rapidly  spinning  the  lamp, 
as  the  regularity  of  the  shape  of  the  filament  does  not  matter. 

In  various  tests  which  have  been  published  from  time  to 
time  readings  have  been  taken  at  every  SOdeg.  all  round  the 


192  THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

lamp.  This  gives  a  mean  value  sufficiently  close  to  the  real 
one  for  most  purposes,  though  in  the  case  of  flat  filaments 
the  mean  value  obtained  in  this  way  may  vary  3  per  cent., 
according  to  the  position  of  the  filament — that  is  to  say, 
whether  the  flat  side  of  the  filament  is  set  towards  the  photo- 
meter at  angles  of  Odeg.,  SOdeg.,  GOdeg.,  &c.,  or  15deg., 
45deg.,  75deg.,  &c.  In  the  former  case  the  mean  of  the 
readings  will  be  about  2  per  cent,  below  the  real  mean,  and  in 
the  latter  case  about  1  per  cent,  above.  Such  differences  as 
these  are,  perhaps,  hardly  worth  troubling  about,  except  in 
the  case  of  special  tests. 

Before  the  lamps  are  tested  they  should  be  thoroughly 
cleaned.  A  dirty  bulb  will  obscure  the  light  very  materially. 
After  testing  they  must  be  sorted  out  according  to  their 
voltage.  If  they  are  loop  lamps,  they  are  finished  except  as 
to  final  cleaning  and  wrapping  in  paper.  The  cleaning  is  best 
done  by  wiping  them  with  soft  paper,  first  wet  and  then  dry. 
This  is  much  more  effective  than  using  a  cloth.  When  finally 
cleaned,  the  lamps  should  not  again  be  touched  with  the 
hand,  or  finger-marks  will  be  left  upon  the  glass. 


CHAPTER  XIV. 


CAPPING. 

IF  the  lamps  are  to  be  capped  the  capping  should  be  done 
after  they  have  been  tested.  Lamps  are  fitted  with  caps  for 
the  purpose  of  providing  a  convenient  and  simple  means  of 
securely  attaching  them  to  a  holder,  and  making  the  necessary 
electrical  connections. 

The  most  usual  form  of  cap  in  this  country  consists  of  a 
short  length  of  brass  tube  which  fits  over  the  neck  of  the 
lamp  and  encloses  the  platinum  wires.  The  platinum  wires 
are  usually  soldered  to  copper  wires,  which  in  turn  are 
soldered  to  the  metal  contact  plates  of  the  cap.  The  cap  is 
filled  up  with  some  kind  of  plaster,  which  holds  it  to  the  lamp 
and  secures  the  contact  plates.  There  are  pins  on  each  side 
of  the  brass  tube  for  securing  the  cap  in  the  bayonet  slot  of 
the  holder.  There  are  numbers  of  different  forms  of  caps 
designed  to  fit  various  patterns  of  holders.  While  it  is  not 
intended  to  discuss  the  merits  of  the  different  varieties,  there 
are  still  a  few  points  to  be  mentioned  in  connection  with  caps 
generally. 

Plaster-of-paris  is  generally  used  to  fill  up  the  space  in  the 
caps.  The  end  of  the  lamp,  as  already  explained  hi  the 
chapter  on  "  Sealing-In,"  is  formed  in  such  a  way  that  the 
plaster  will  have  a  firm  hold  upon  it.  Different  samples  of 
plaster-of-paris  are  found  to  vary  very  much.  Some  will  set 
much  more  quickly  and  become  much  harder  than  others. 
Good  plaster-of-paris  sets  quickly  and  expands  in  setting. 
The  expansion  in  setting  of  some  samples  will  swell  the  brass 
tube  of  the  cap  to  such  an  extent  that  it  will  no  longer  go  into 
the  holder  if  it  was  of  a  fairly  close  fit  before  the  plaster  was 

o 


194         THE    INCANDESCENT    LAMP    AND   ITS    MANUFACTURE. 

put  in.  The  addition  of  slaked  lime  to  the  plaster  will,  how- 
ever, reduce  or  prevent  the  expansion,  according  to  the  propor- 
tion used,  which  must  be  regulated  by  the  quality  of  the 
plaster.  The  Author  has  also  found  that  the  addition  of  a 
small  percentage  of  dextrine  to  the  plaster-of-paris  makes  it 
very  much  harder. 

Another  plaster  sometimes  used  in  caps  is  made  of  litharge, 
which  is  made  into  a  paste  with  glycerine.  This  plaster, 
which  sets  very  hard,  is  very  heavy  and  more  expensive  than 
the  plaster-of-paris. 

Current  should  never  be  put  on  the  lamps  until  the  plaster 
is  quite  dry.  Plaster-of-paris  takes  at  least  four  days  to  dry 
spontaneously.  If  the  lamp  is  lighted  before  the  plaster  is  dry 
a  perceptible  amount  of  current  will  pass  through  the  plaster 
in  the  cap,  and  one  of  the  copper  wires  will  be  electrolised. 
The  Author  has  known  cases  where  the  fine  copper  wire  has 
been  eaten  through  and  the  connection  with  the  lamp  destroyed 
in  this  way.  Plaster-of-paris  can,  however,  be  easily  dried  by 
heat,  so  that  in  a  couple  of  hours'  time  after  mixing  it  will 
conduct  no  current.  Resin  should  be  used  as  a  flux  for  sol- 
dering the  wires  to  the  contact  plates.  When  a  lamp  holder 
is  discovered  to  be  very  hot,  as  is  sometimes  found  to  be  the 
case,  the  heating  is  often  caused  by  a  current  passing  though 
the  plaster  of  the  cap,  but  is  generally  and  often  erroneously 
attributed  to  a  bad  contact  between  the  holder  and  the  cap. 
The  trouble  is  sometimes  due  to  the  too  abundant  use  of  an 
acid  flux  in  soldering,  the  excess  remaining  in  the  plaster. 


CHAPTER   XV. 


EFFICIENCY  AND  DURATION. 

THE  Author  has  adhered  to  the  expression  "  watts  per 
candle-power"  in  preference  to  that  of  "candles  per  watt," 
as,  although  the  latter  is  the  more  correct,  the  former  has  the 
advantage  of  being  the  one  generally  adopted,  doubtless  be- 
cause it  is  more  convenient.  It  is  easier  to  quickly  grasp  the 
meaning  of  a  certain  number  of  watts,  and  perhaps  a  fraction 
added,  than  of  a  particular  fraction  of  a  candle-power. 

It  will  be  readily  understood  from  what  has  already  been 
said  that  no  lamp  can  be  said  to  be  of  or  to  have  a  greater  or 
less  efficiency  than  another.  One  lamp  may  be  run  at  a 
greater  efficiency  than  another,  and  may  then  be  said  to  be  «t 
a  greater  efficiency,  but  it  possesses  in  itself  no  greater  efii- 
ciency,  as  the  second  lamp  can  be  equally  well  run  at  the 
higher  efficiency. 

The  higher  state  of  efficiency  simply  means  that  the  fila- 
ment is  at  a  higher  temperature.  Any  lamp  may  be  run  at 
any  efficiency  or  temperature  up  to  its  breaking  point.  With 
incandescent  carbon  a  certain  temperature  means  a  certain 
colour  of  light  and  a  definite  efficiency,  either  in  watts  per 
candle-power  or  in  proportion  of  luminous  radiation  to  total 
radiation.  Whether  the  same  thing  can  be  said  of  all  incan- 
descent substances  does  not  yet  appear  to  be  definitely  settled. 
Certain  of  the  metallic  oxides  may,  perhaps,  prove  exceptions. 
The  well-known  Welsbach  gas  burner  seems  to  suggest  such 
an  exception,  but  the  Author  does  not  know  of  any  tests  upon 
this  point.  Certainly,  to  the  unaided  eye,  the  colour  of  the 
light  of  a  Welsbach  mantle  is  absolutely  different  from  that  of 
a  gas  flame,  though  the  temperature  of  the  particles  of  carbon 

o  2 


196          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

in  a  gas  flame  might  be  supposed  to  be  much  the  same  as  that 
of  the  Welsbach  mantle.  Certainly  neither  can  be  hotter  than 
the  gas  flame.  Possibly,  however,  the  particles  of  carbon  are 
very  far  below  the  temperature  of  the  flame.  These  particles 
of  carbon  have  a  very  great  emissivity,  and  may,  perhaps, 
radiate  heat  and  light  so  fast  that  they  never  approach  the 
actual  temperature  produced  by  the  combustion  of  the  gas. 
On  the  other  hand,  the  Welsbach  mantle,  having  a  vastly 
inferior  emissivity,  is  raised  nearly  to  the  actual  temperature 
of  the  flame.  The  emissivity  of  the  Welsbach  mantle  is 
extremely  low.  One  which  the  Author  measured  gave  when 
new  less  than  eight  candles  per  square  inch  of  surface.  The 
carbon  in  a  gas  flame  gives  many  times  this  amount  of  light 
per  square  inch.  Accurate  tests  with  a  spectro-photometer 
might  settle  this  question.  If  the  material  of  the  Welsbach 
mantle  is  only  at  the  same  temperature  as  the  carbon  particles 
in  a  gas  flame,  then  it  must  have  the  property  of  selective 
radiation,  and  may,  therefore,  be  found  to  have  a  greater  light- 
giving  efficiency  than  carbon. 

In  the  case  of  using  such  oxides  as  the  light-giving  portion 
in  electric  lamps,  one  thing  is  apparent :  that  the  extent  of 
radiating  surface  will  have  to  be  very  much  greater  than  that 
of  a  carbon  filament  for  the  same  amount  of  light  if  at  the 
same  temperature. 

The  only  way  to  ascertain  the  relative  superiority  of  different 
incandescent  lamps  is  to  run  them  all  at  a  constant  pressure 
and  all  at  the  same  efficiency  at  the  start.  They  must  then 
be  tested  at  periods  of,  say,  fifty  hours  at  the  same  pressure  as 
the  original  test,  in  order  to  find  the  diminution  in  candle- 
power.  Such  tests  might  be  called  "  deterioration  "  tests. 
Thus,  a  lamp  losing  10  per  cent,  in  candle-power  in  400  hours- 
is  a  better  lamp  than  one  losing  that  amount  in  300  hours. 

Life  tests,  pure  and  simple,  are  worthless.  A  great  many 
carefully-ascertained  life  tests  of  different  lamps  have  at  various 
times  been  reported.  Such  tests  are,  however,  of  no  value 
unless  the  actual  tests  of  candle-power  of  the  lamp  at  different 
periods  of  its  life  are  also  given.  It  is  also  equally  necessary 
to  know  within  what  limits  the  difference  of  potential  was 
maintained  throughout  the  run,  as  well  as  the  length  of  time 
during  which  it  was  above  or  below  the  normal. 


EFFICIENCY    AND    DURATION.  197 

All  lamps  fall  off  in  candle-power  when  supplied  with  a  con- 
stant number  of  watts.  Lamps  usually  fall  off  in  candle- 
power  from  the  commencement  of  their  active  life,  when  sup- 
plied with  current  at  a  constant  pressure.  The  deterioration  in 
this  case  is,  however,  sometimes  masked  during  the  first  hun- 
dred hours  or  so  by  the  resistance  of  the  filament  falling.  A 
greater  amount  of  power  is  therefore  supplied  to  the  lamp, 
which  may,  in  consequence,  give  even  more  light  after  a  hun- 
dred hours  than  it  did  at  the  first.  Such  behaviour  is,  how- 
ever, the  exception  rather  than  the  rule,  and  seldom  occurs  in 
flashed  lamps. 

What  amount  of  deterioration  should  be  allowed  before  a 
lamp  is  cast  aside  it  is  difficult  to  say,  and  depends  upon  the 
circumstances  of  each  individual  case.  As  long  as  a  lamp 
gives  sufficient  light  in  its  particular  situation  there  is  no  need 
to  change  it.  A  new  lamp  may  be  fixed  over  a  writing  table, 
and  after  a  hundred  hours  may  be  found  to  be  no  longer  bright 
enough ;  it  is  therefore  taken  down,  but  it  will  do  perfectly  well 
in  some  other  situation — say,  in  a  bedroom  or  passage — for  a 
further  and  longer  period.  The  price  of  the  lamp  determines 
as  much  as  anything  the  period  at  which  it  is  thrown  away. 
The  cost  of  a  candle-power  hour  for  the  actual  amount  of 
light  given  increases  as  the  light  of  the  lamp  diminishes.  In 
order  that  the  total  cost  of  maintaining  a  given  amount 
of  light  may  be  a  minimum,  it  is  necessary  to  renew  the 
lamps  at  certain  definite  times.  The  length  of  tune  each 
lamp  is  used  depends  in  such  case  upon  the  price  of  the  lamp, 
the  price  of  the  power,  and  the  rate  of  deterioration  of  the 
lamp.  As  the  last  of  these  conditions  is  variable  even  with 
lamps  of  the  same  make,  it  is  impossible  to  fix  the  period 
under  any  conditions  of  price  of  lamps  and  power.  Lamps 
in  ordinary  use  cannot  be  tested  every  fifty  hours,  and  it, 
therefore,  happens  that  they  remain  in  use  as  long  as  they 
give  light  enough. 

The  length  of  time  taken  by  lamps  of  the  same  make  to 
deteriorate  in  candle-power  by  a  certain  percentage  depends 
very  much  on  their  treatment.  If  the  pressure  is  maintained 
constant,  say  within  2  per  cent,  on  either  side  of  the  normal, 
the  useful  life  will  be  longer  than  if  the  pressure  is  allowed 
to  vary  by  5  per  cent.  The  pressure  at  which  electricity  is 


198 


THE    INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


supplied  by  central  stations  is  anything  but  constant.  The  very 
best  supplies  are  not  to  be  relied  upon  to  keep  within  5  per 
cent,  on  either  side  of  the  normal,  while  some  extensive  supply 
systems  do  not  keep  within  10  per  cent.  The  alternating 
systems  seem  to  be  more  difficult  to  regulate  than  the  direct 
current.  Deterioration  tests  of  lamps  of  the  same  make  on  the 


180 


»60 


«  100 

c 
c 

§ 
80 


60 


40  _ 


108 


112 


92  96  100  104 

Percentage  of  Volts. 

FIG.  64. — Curve  showing   Percentage  of   Normal   Candle-Power  at  any 
given  Percentage  of  the  Normal  Volts. 

different  supply  systems  would  be  very  interesting  and  instruc- 
tive, and  would  go  far  towards  settling  the  dispute  as  to  the 
relative  merits  of  the  various  systems  in  use. 

In  order  to  show  the  importance  of  the  constancy  of  the 
pressure,  Fig.  64  is  given.  It  shows  approximately  the  per- 
centage variation  in  candle-power  for  any  given  percentage 


EFFICIENCY    AND    DURATION.  199 

variation  in  the  volts  over  a  range  of  10  per  cent,  above  and 
below  the  normal.  It  will  be  found  to  be  about  correct  what- 
ever be  the  temperature  or  efficiency  of  the  lamp  at  normal 
voltage.  The  curve  A  gives  the  results  when  the  filament  is 
constant  in  resistance  over  the  range  of  10  per  cent,  in  volts. 
The  dotted  curve  B  gives  the  result  if  the  filament  uniformly 
falls  off  in  resistance  by  5  per  cent,  for  an  increase  of  10  per 
cent,  in  the  volts — that  is  to  say,  the  resistance  falls  1  per  cent, 
for  a  2  per  cent,  rise  in  volts,  or  2  per  cent,  for  a  4  per  cent, 
rise,  &c.  As  a  rule  the  values  lie  nearer  B  than  A.  It  will 
be  seen  that  a  slight  increase  in  the  pressure  causes  the 
candle-power  to  rise  greatly,  while  a  slight  decrease  means  a 
great  falling  off  in  the  light. 

The  falling  off  in  the  candle-power  of  lamps  is  due  to  par- 
ticles of  carbon  being  thrown  off  the  filaments.  This  action 
reduces  the  light  of  the  lamp  in  three  ways.  Firstly,  the 
particles  of  carbon  form  a  coating  on  the  glass  of  the  lamp, 
and,  therefore,  obscure  the  light  given  by  the  filament. 
Secondly,  the  nature  of  the  surface  of  the  filament  is  altered, 
and  its  emissivity  is  increased  so  that  it  is  at  a  lower  tempera- 
ture. Thirdly,  the  resistance  of  the  filament  is  increased,  so 
that  it  takes  less  current,  and  is  again,  on  this  account,  at  a 
still  lower  temperature.  Thus,  not  only  does  the  filament 
give  less  light,  but  a  great  proportion  of  what  it  does  give  is 
prevented  from  getting  outside  the  bulb.  In  order  to  find 
out  what  proportion  of  the  total  loss  of  candle-power  might 
be  due  to  these  different  causes  the  Author  made  the  follow- 
ing somewhat  extreme  test : — 

A  lamp  was  tested  at  about  2  watts  per  candle-power.  It 
took  51-7  volts  and  gave  55  candle-power  for  a  consumption 
of  107-7  watts.  It  was  then  run  at  that  voltage  until  it  was 
much  blackened,  and  it  was  then  tested  again.  At  51-7  volts 
it  gave  only  12  candles,  and,  owing  to  rise  in  resistance,  it 
took  only  100  watts.  The  total  loss  of  candle-power  was, 
therefore,  55  -  12  =  43,  or  78  per  cent.  In  order  to  find  out 
how  this  loss  was  made  up,  the  filament  was  taken  out  of  the 
blackened  bulb  and  put  into  a  new  one.  The  air  was  then 
exhausted  to  the  same  degree  as  before,  showing  a  very  good 
vacuum  by  the  spark  test.  The  lamp  was  then  again  tested. 
At  51-7  volts  it  took  100  watts,  and  gave  30  candles.  It  was 


200 


THE   INCANDESCENT   LAMP    AND    ITS    MANUFACTURE. 


then  run  a  little  brighter,  so  as  to  take  the  original  power  of 
107-7  watts,  and  it  gave  42  candles  and  took  52-8  volts.  The 
loss  of  light  due  to  the  blackened  bulb  was,  therefore,  80  -  12 
=  18  candles  =  60  per  cent. 

The  loss  due  to  increase  in  emissivity  is  given  by  the  de- 
crease in  candle-power,  when  the  same  amount  of  power  was 
supplied  to  the  filament.  The  loss  due  to  change  in  emis- 
sivity is,  therefore,  55  -  42  =  13  candles  =  23-6  per  cent. 

The  loss  due  to  change  in  resistance  is  given  by  the  differ- 
ence in  candle-power  when  the  filament  was  supplied  with  the 
original  number  of  watts,  and  when  supplied  with  the  original 
number  of  volts ;  because,  had  the  resistance  not  gone  up, 
the  original  number  of  volts  would  have  given  also  the 
original  number  of  watts.  The  loss  due  to  increased  resist- 
ance is,  therefore,  42  -  30  =  12  candles  in  55  =  21-8  per  cent. 
The  rise  in  resistance  is  about  8  per  cent.  The  loss  due  to 
change  in  emissivity,  plus  that  due  to  change  in  resistance, 
therefore  =  13  + 12  =  25  candles  =  45-5  per  cent. 

This  result  is  also  shown  by  the  difference  between  the 
original  candle-power  and  that  at  the  original  volts  when  the 
filament  is  in  the  new  bulb,  i.e.,  55-30  =  25  candles.  It  is, 
therefore,  apparent  that  the  filament  at  the  original  volts  gives 
25  c.p.  less  than  it  gave  at  first :  it  gives  only  30  candles,  or 
54'5  per  cent,  of  its  original  candle-power. 

We  have  already  seen  that  the  blackened  glass  obstructs  60 
per  cent,  of  the  light  of  the  filament ;  therefore,  of  this  54-5 
per  cent,  of  light  given  by  the  filament  60  per  cent,  is  stopped 
by  the  blackened  bulb. 

60  per  cent,  of  54-5  per  cent.  =  32-6  per  cent.  The  total  re- 
duction in  the  light  of  the  lamp  is  therefore  made  up  in  the 
following  way : — 


Less  light  given  by  filament,  owing  to 

Candles. 

Per  cent. 

(1)  Increase  in  emissivity 

13 

23-6 

(2)             ,  ,         resistance 

12 

21-8 

Total  (1)  and  (2) 

25 

45-4 

(3)  Stopped  by  blackened  glass. 

18 

32-6 

Total  reduction   

43 

78-0 

EFFICIENCY   AND    DURATION.  201 

The  greater  part  of  the  decrease  in  the  light  of  the  lamp  is, 
therefore,  owing  to  the  filament  giving  less  light.  Of  that 
which  it  does  give  only  a  part  gets  through  the  blackened  bulb. 

Though  the  above  test  is  perhaps  an  extreme  one,  the 
lamp  having  been  very  much  overrun,  the  same  kind  of  result 
undoubtedly  occurs  in  lamps  run  at  ordinary  temperatures. 

The  black  deposit  on  the  bulb  of  a  lamp  stops  not  only 
much  of  the  light  radiation,  but  also  much  of  the  heat.  The 
result  is  that  a  blackened  lamp  often  gets  extremely  hot. 

The  effect  of  a  fall  in  the  resistance  of  a  lamp  is  strikingly 
shown  in  the  following  test : — An  unflashed  lamp  had  just  been 
tested  at  50  volts.  It  gave  12-5  candles,  and  took  0'82 
ampere,  and,  therefore,  41  watts,  and  its  resistance  was  61 
ohms.  It  was  then  accidentally  run  for  an  instant  at  100 
volts.  The  testing  was  being  done  on  a  100-volt  circuit,  and 
the  resistance  in  series  with  the  lamp  was  momentarily  short- 
circuited.  The  bulb  was  in  consequence  blackened,  and  the 
filament  had  lost  its  shiny  appearance.  It  was,  therefore, 
expected  that  a  great  diminution  in  candle-power  would  be 
observed  at  50  volts.  The  resistance  of  the  filament  had, 
however,  been  so  much  reduced  as  to  more  than  counteract 
the  dulling  effects.  The  lamp  tested  50  volts,  1-03  ampere, 
22  c.p.,  and,  therefore,  51-5  watts  and  48-5  ohms.  The  resist- 
ance had  gone  down  20  per  cent.,  so  that  the  lamp  took  so 
much  more  power  than  it  did  at  the  first  that  the  increase  in 
emissivity,  which  undoubtedly  occurred,  was  not  sufficient  to 
prevent  a  great  increase  in  the  temperature  of  the  filament ; 
and  the  blackening  of  the  bulb,  though  great,  was  also  not 
sufficient  to  mask  the  effect  of  the  great  increase  in  the  light 
given  by  the  filament. 

If  a  filament  could  be  found  whose  resistance  gradually  fell 
as  its  emissivity  increased  and  the  bulb  blackened,  a  lamp  of 
uniform  candle-power  or  of  uniform  watts  per  candle-power 
throughout  its  life  might  be  possible.  Such  a  lamp  would  not 
have  so  long  a  life  as  one  of  which  the  resistance  increases, 
but  it  would  have  a  more  useful  life,  and  would  automatically 
end  its  career  at  the  proper  time.  Unfortunately,  the  fila- 
ments which  are  usually  found  to  fall  in  resistance  do  so  during 
the  first  part  of  their  life,  when  they  should  remain  constant ; 
and  when  they  should  be  going  down  they  increase. 


CHAPTER  XVI. 


RELATION  BETWEEN  LIGHT  AND  POWER. 

IT  has  already  been  stated  that  the  light  given  out  by  an 
incandescent  filament  increases  at  a  much  greater  rate  than 
the  power  spent  in  the  filament.  The  relation  of  the  light  to 
the  power,  or  to  the  pressure  or  the  current,  has  been  the 
subject  of  much  study  and  speculation.  Many  and  various 
formulae  have  been  given  by  different  investigators.  Some 
of  them  are  nearly  correct  when  applied  to  certain  lamps, 
but  are  found  to  be  quite  useless  when  applied  to  others. 
No  formula  yet  given  appears  to  satisfy  all  cases.  The  reason 
probably  lies  in  the  imperfections  of  the  lamps.  The  filaments 
of  different  lamps  vary  in  their  behaviour  at  different  tem- 
peratures, and  the  vacuums  also  vary. 

In  1882  Prof.  A.  Jamieson  showed  that  the  candle-power  of 
incandescent  lamps  was  approximately  proportional  to  the  sixth 
power  of  the  pressure,  and,  therefore,  to  the  sixth  power  of 
the  current,  since  the  resistance  of  the  filament  when  incan- 
descent does  not  alter  much. 

The  most  recently  published  tests  have  been  carried  out, 
under  the  direction  of  Prof.  Ayrton,  at  the  Central  Institution 
of  the  City  and  Guilds  of  London,  and  were  published  in  part 
in  Tlie  Electrician  of  July  15,  1892.  These  tests  are  valuable, 
as  they  appear  to  have  been  carried  out  with  great  care  to 
eliminate  various  errors,  and  the  observations  were  made  by 
three  observers. 

It  is  shown  by  these  tests  that  there  is  a  definite  relation 
between  the  logarithms  of  the  candle-power  and  of  the  volts, 
amperes,  or  watts  respectively  of  each  lamp.  As,  however, 
the  tests  referred  to  are  not  carried  above  the  temperature 


204 


THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


of  about  4  watts  per  candle-power,  the  Author  introduces 
here  some  tests  which  he  has  made  up  to  a  much  higher 
temperature." 


Lamp  B. 

Lamp  D. 

C.P. 

Volts. 

Amps 

Watts. 

Ohms 

aV» 

C.P. 

Volts. 

Amps 

Watts. 

Ohms 

aV» 

5-3 

29-0 

1-13 

32-6 

25-7 

5-15| 

2-0 

30-0 

1-03 

30-9 

29-1 

2-33 

6-2 

30-0 

1-15 

34-5 

26-1 

6-2  : 

3-0 

32-0 

1-1 

35-2 

29-1 

3-22 

8'8 

32-5 

1-23 

40-0 

26-4 

9-52 

4-0 

33-0 

1-15 

37-9 

28-7 

3-92 

10-6 

33-0 

1-2$ 

42-2 

25-8 

10-3 

5-0 

34-5 

1-2 

41-4 

28-8 

5-05 

12-5 

34-0 

1-31 

44-5 

26-0 

12-07 

6-0 

36-0 

1-24 

44-7 

29-0 

6-34 

14-2 

35-0 

1-34 

4tJ-9 

26-1 

1418 

7-0 

37-0 

1-27 

47'0 

29-2 

7'4 

16'0 

36-0 

1-36 

48-9 

26-5 

16-4 

8'0 

37-5 

1-3 

48-7 

2b-9 

7-97 

177 

36-5 

1-39 

50-7 

26-3 

17-65 

9-0 

38-0 

1-33 

50-5 

28-6 

8-5 

22*2 

38-0 

1-45 

55-1 

26-2 

22-0 

10-0 

39-0 

1-36 

53-0 

28-7 

10-0 

26-5 

39-5 

1-49 

58-8 

26-5 

26-8 

12-5 

40-2 

1-4 

56-3 

28-7 

11-68 

35-4 

42-0 

1-58 

66-4 

26-6 

37-4 

15-0 

41-8 

1-45 

60-5 

28-6 

14-5 

44-0 

43-5 

1-66 

72-1 

26-2 

45-0 

20-0 

43-7 

1-51 

C6-0 

29-0 

18-4 

53-0 

45-0 

1-7 

76-5 

26-4 

51-4 

25-0 

45-5 

1-58 

72-0 

28-8 

24-0 

62-0 

46-0 

1-75 

80-5 

26-4 

61-3 

30-0 

47-0 

1-62 

76  -5 

29-0 

27-6 

71'0 

46-8 

1-78 

83-5 

26-4 

66-8 

35-0 

48-5 

1-H8 

81-5 

28-9 

32-6 

80-0 

47-8 

1-81 

86-5 

26-4 

74-6 

45-0 

51-2 

1-77 

90  ''-> 

28-9 

44-2 

106-0 

50-5 

1-9 

96-0 

26-6 

100-0 

f.0'0 

52-2 

1-85 

1)4-5 

28-2 

48-9 

124'0 

52-0 

1-94 

10  1  -0 

26-8 

117-8 

60-0 

54-2 

1-88 

102-0 

27-8 

607 

142-0 

53-0 

1-98 

105-0 

26-8 

130-0 

70-0 

55-4 

1-92 

106-2 

28-8 

68-0 

159-0 

55-0 

2-05 

112-8 

26-8 

159-0 

80-0 

57-0 

1-98 

113-0 

28  -8 

80-0 

177-0 

56-0 

2-07 

116-0 

27-0 

174-5 

PO-O 

57-8 

2-01 

116-3 

28-8 

86-5 

195-0 

58-0 

•2'\  3 

123-6 

27-2 

210-0 

100-0 

59-6 

2-08 

124-0 

28-8 

102-0 

212-0 

60-0 

2-2 

132-0 

27-3 

2.i2-0 

120-0 

62-0 

2-17 

134-5 

28-6 

127-0 

230-0 

61-8 

2-22 

137-2 

27  -H 

296-0 

140-0 

64-8 

2-2o 

145-0 

28-8 

166-0 

248-0 

65-0 

2-3 

149-5 

28-3 

389-0 

160-0 

68-0 

2-35 

160-0 

28-4 

2U-0 

"... 

180-0 

69-0 

2-38 

164-0 

28-9 

230-0 

... 

200-0 

72-o 

2-49 

181-0 

2»'l 

300-0 

250-0 

80'0 

2'7 

21H-0 

29-6 

518-0 

... 

... 

... 

295-0 

90-0 

2-8 

252-0 

32-2 

icoo-o 

The  lamps  B,  C,  D,  are  of  Edison- Swan  manufacture,  and 
lamp  A  is  an  unflashed  amyloid  filament  lamp.  The  tests  of 
A  and  C  were,  however,  not  carried  very  far. 

Fig.  65  shows  the  curves  of  watts  and  candle-power  up  to 
about  2  watts  per  candle-power,  and  Fig.  66  shows  the  results 
of  lamps  B  and  D  run  up  to  their  breaking  points.  The  dotted 
lines  show  the  watts  per  candle-power. 

It  will  be  noticed  that  both  the  curves  (Fig.  66)  turn  over  at 
the  higher  readings.  This  is  caused  by  the  disintegration  of 
the  filament  and  its  consequent  change  in  emissivity  and  re- 
sistance, and  by  the  blackening  of  the  bulb.  The  direction  of 
the  curves  above  the  point  where  rapid  disintegration  begins 
depends,  among  other  things,  upon  the  length  of  time  during 


FIG.  65. — Curves  showing  Relation  of  Candle-Power  and  Watts  for  Four 
Lamps,  A,  B,  C,  D. 


320 


280 


40       80       120       160      200      240      280 
Watts. 

FIG.  66. — Curves  showing  Relation  of  Candle-Power  and  Watts  for  Lamps 
B  and  D,  up  to  their  Breaking  Points. 


206          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

which  the  lamp  is  kept  at  each  temperature  while  the  obser- 
vations are  being  made.  The  curve  of  a  lamp  which  is  kept 
at  1  watt  per  candle-power  for  some  time  may,  for  instance,  be 
made  to  bend  over  so  much  that  it  will  never  reach  0-9  watt 
per  candle-power,  as  the  lamp  will  break  before  arriving  there. 
It  will  be  seen  that  lamp  B  attains  a  much  higher  efficiency 
than  lamp  D.  The  observations  at  the  higher  readings  in  each 
case  were,  however,  made  as  quickly  as  possible. 

Fig.  67  shows  the  relation  between  the  logarithms  of  the 
candle-power  and  of  the  volts  and  watts  respectively.  Up  to 
the  point  where  the  lamp  begins  to  rapidly  deteriorate  it  will 
be  noticed  that  the  observations  in  this  figure  lie  nearly  in 
straight  lines  —  sufficiently  so  to  suggest  that  they  ought  to 
be  exactly  in  straight  lines.  Assuming  that  to  be  the  case, 
the  expression  C.P.  =  aVn,  where  C.  P.  =  candle-power  and 
V  =  volts,  a  and  n  being  constants,  would  enable  us  to  find 
the  candle-power  at  any  voltage  when  the  values  of  a  and  n 
are  known. 

In  order  to  find  the  value  of  a  and  n  for  any  lamp,  two 
observations  at  least  must  be  made. 

If  c.p.  =  candle-power  at  the  lower  reading, 

,,     v     =  volts  „  ,,  „ 

,,  C.P.  =  candle-power  ,,         higher        ,, 
V    =  volts 


then  „,  --  «£. 

log 


and  «- 

v 


The  observations  from  which  the  value  of  a  and  n  are 
obtained  must  be  made  with  the  greatest  care,  and  should 
be  as  wide  apart  as  possible.  A  very  slight  error  in  the  test 
will  give  a  wrong  value  to  the  exponent  n,  and  will  make  the 
calculated  results  quite  wrong. 

In  the  case  of  lamp  B,  taking  the  readings  of  30  volts 
6-2  c.p.,  and  55  volts  159  c.p.,  the  value  of  n  is  found  to  be 
5-35  and  a  =  7-76xlO-8. 

In  the  same  way  for  lamp  D,  at  39  volts  10  c.p.  and  57 
volts  80  c.p.,  7i  =  5-51  and  a  =  1-695  x  10~s. 


RELATION    BETWEEN    LIGHT   AND    TOWER. 


207 


7  19  21 

Log    Volts,  or  Log.  Watts. 


FEO.  67. — Diagram  showing  Relation  of  the  Logarithms  of  the  Candle- 
Powers  to  the  Logarithms  of  the  Volts  and  of  the  Watts  respectively  of 
Lamps  B  and'D. 


208 


THE   INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 


In  Figs.  68  and  69  the  dotted  curves  show  the  values  cal- 
culated on  the  above  basis.  The  crosses  mark  the  observed 
values.  Over  the  lower  portion  of  the  curves  the  observed 
and  the  calculated  values  lie  very  close  together.  In  Fig.  69 
it  will  be  seen  that  the  curve  B,  representing  the  actual  obser- 
vations, definitely  takes  a  different  direction  from  B',  represent- 
ing the  calculated  values  at  about  180  c. p.,  or  at  an  efficiency 


/ 

B; 
* 

r 
( 
/ 

D/ 

/ 
/ 

; 
i 
* 

V 

/ 
/ 
/ 
/ 
/ 

i 
i 
i 
k 

/ 

1 

1 

/ 

/ 
1 

i 
i 

r 
/ 

/ 
/ 

/     ,' 

i          / 
'          / 

1 

/ 

i 

/ 
/ 
j 
/ 
/ 

L, 

/ 
/ 
/ 
/ 

V 

X 

0                  30                 40                  60                 6( 

FIG.  68. — Lower  Part  of  Curves  of   Candle-Power  and  Volts  for  Lamps 

B  and  D. 


of  0*65  watt  per  candle.  The  curve  D  separates  from  D'  at 
100  candles,  corresponding  to  an  efficiency  of  1*25  watts  per 
candle-power  only.  This  would  seem  to  show  that  lamp  D 
was  an  inferior  one  to  lamp  B. 

That  great  care  is  necessary  in  making  the  tests  for  a  and  n 
can  be  seen  in  the  case  of  lamp  B.  If  two  observations  only 
were  made,  and  these  were,  say,  those  at  80  volts  6'2  c.p.  and 


RELATION    BETWEEN    LIGHT    AND    POWER. 


209 


35  volts  14*2  c. p.,  the  value  of  n  would  come  out  at  5-38,  which 
is  very  nearly  right,  these  two  observations  lying  almost 
exactly  on  the  straight  line  in  Fig.  66.  If,  however,  the  tests 
were  those  of  32-5  volts  8-8  c.p.,  and  34  volts  12*5  c.p.,  the 
one  reading  of  candle-power  being  a  little  low  and  the  other  a 


320, — 


280 


40  60 

Volts 


100 


FIG.  69. — Curves  of  Candle-Power  and  Volts  for  Lamps  B  and  D  up  to 
their  Breaking  Points. 


little  high,  the  value  of  n  would  be  7'9,  and  the  calculated 
results  would  be  quite  wrong,  these  two  observations  lying 
one  a  little  above  and  the  other  a  little  below  the  line  in 
Fig.  67,  where  they  are  indicated  by  the  dotted  circles,  while 
the  observations  from  which  the  correct  values  are  obtained 
are  shown  by  the  full  circles. 


210          THE    INCANDESCENT    LAMP    AND    ITS    MANUFACTURE. 

It  is  probable  that  the  relation  of  candle-power  to  watts  is 
a  more  constant  one  among  different  lamps  than  that  of 
candle-power  to  volts  or  amperes.  In  the  latter  case  the 
degree  of  constancy  of  the  resistance  of  the  filament  through- 
out the  range  of  the  test  has  an  effect  upon  the  form  of  the 
curve,  whereas  in  the  former  case  it  has  no  such  effect. 

Throughout  this  work  the  Author  has  adopted  the  efficiency 
of  4  watts  per  candle-power  as  the  standard.  This  efficiency 
may  be  regarded  as  the  highest  ordinarily  met  with  in  lamps 
in  actual  use.  At  4  watts  a  lamp  looks  bright  as  compared 
with  a  gas-flame.  Although  lamps  are  sold  marked  at  3-5 
and  3,  or  even  a  less  number  of  watts  per  candle,  they  very 
soon  arrive  at  4  watts  when  in  use.  The  mean  efficiency  of 
any  number  of  lamps  in  actual  use  in  any  installation,  other 
than  quite  a  new  one,  is  probably  quite  as  low  as  5  watts  per 
candle-power. 

At  a  temperature  of  4  watts  per  candle-power,  it  has  been 
estimated,  by  actual  experiment,  that  the  proportion  of  lumi- 
nous radiation  to  total  radiation  in  an  incandescent  lamp  is 
about  5  per  cent,  only,  while  the  proportion  in  ths  case  of  the 
arc  lamp  is  about  10  per  cent.  In  Fig.  66  it  will  be  noticed 
that,  by  running  lamp  B  up  to  its  breaking  point,  an  efficiency 
of  0*6  watt  per  candle-power  is  attained,  equal  to  nearly 
seven  times  greater  than  at  4  watts  per  candle-power.  It 
might,  therefore,  be  expected  that  the  proport  on  of  luminous 
radiation  to  total  radiation  at  this  temperature  was  seven 
times  that  at  4  watts  per  candle-power,  or  35  per  cent.,  which 
would  be  far  greater  than  that  of  the  arc  lamp.  This,  how- 
ever, is  not  at  all  the  case.  The  candle-power  is  in  no  way 
a  measure  of  the  energy  of  the  luminous  radiation  when 
comparing  lights  of  different  colours.  Although  by  increasing 
its  temperature  we  get  an  enormously  increased  efficiency  as 
regards  candle-power  per  watt  expended  in  the  filament, 
the  ratio  of  the  luminous  energy  to  the  total  energy  of  radia- 
tion does  not  increase  at  anything  like  the  same  rate.  The 
increasingly  refrangible  rays,  which  are  produced  as  the 
temperature  is  raised,  add  little  to  the  light  as  measured  by 
the  photometer,  while  they  require  comparatively  a  large 
amount  of  power  for  their  production,  Even  under  the 


RELATION    BETWEEN    LIGHT    AND    POWER.  211 

>s. 

extreme  conditions  of  O6  watt  per  candle-power  the  pro- 
portion of  luminous  radiation  to  total  radiation  of  the  in- 
candescent lamp  probably  falls  short  of  that  attained  in  the 
arc  lamp. 

The  ideal  lamp  is  one  from  which  the  radiation  is  wholly 
luminous.  The  carbon  incandescent  lamp  is,  therefore,  very 
far  indeed  from  attaining  the  ideal  standard.  A  more 
efficient  lamp  must  evidently  be  looked  for  in  another 
direction.  The  beautiful  experiments  shown  by  Mr.  Nikola 
Tesla  would  seem  to  point  to  a  direction  in  which  it  may 
possibly  be  found,  but,  as  yet,  illumination  by  means  of 
vacuum  tubes  has  not  reached  the  efficiency  of  the  incan- 
descent lamp,  and  is  far  behind  it  in  point  of  practicability. 

In  the  preceding  pages  the  Author  has  not  touched  upon 
the  History  of  the  Incandescent  Lamp  or  the  labours  of  the 
men  to  whom  the  world  is  indebted  for  bringing  it  to  a  state 
of  perfection  such  that  it  has  become  an  article  of  everyday 
use.  In  the  first  rank  of  these  are  the  names  of  Swan, 
Edison,  Weston,  and  Lane-Fox  ;  while  following  them  come 
a  host  of  others,  who  have  materially  assisted  in  making  the 
lamp  what  it  is  to-day.  In  a  book  of  this  size  it  would  be 
impossible  to  give  credit  to  each  for  his  share  in  the  work, 
even  if  it  could  be  ascertained.  The  manufacture  of  the 
incandescent  lamp  involves  so  many  and  various  problems, 
and  there  have  been  so  many  workers  in  the  field,  that  many 
inventions  in  connection  with  it  have  been  independently 
made  by  different  individuals. 


INDEX  TO  CONTENTS. 


PAGE 

Air  Film  in  Lamps         162 

„       „     „  Pumps  164 

Amyloid,  Composition  of            ..           ...         ...         ...         ...          ..  14 

Annealing  Glass  ...          ...         ...         .- 123 

Ayrton -Tests  of  Lamps            ...  203 

Bamboo,  Filaments  Prepared  from      ...         ...         ...         ...  19 

Blackening  of  Lamps      ...                       ..  199 

Blow-Pipe  for  Glass-Blowing     110,  111,  113 

„       „      Air  Supply  for          ...         ...         ...         ...         ...         ...  115 

„       „     for  Pump-Room        169 

Bulb  Making  from  Pot  Glass     106 

„         .,        from  Tubing         116 

,    Moulds  107 


Candle-power       190 

„          „       of  Flat  Filaments  191 

„       Falling  off  of       199 

Increased  by  Fall  of  Resistance  197,201 

„          „       Rise  and  Fall  of,  with  Variation  in  Pressure  .  ...       198 

Caps,  Plaster  or  Cement  for  Attaching  194 

Carbon,  Disintegration  of  ...         ...         ...         ...          ...         ...3,  199 

„       Emissivity  of  Different  Varieties  of 4,  63 

.,       Resistance  of       62 

„        Volatilisation  of  3 

Carbonisation       ...         ...         ...         ...         ...         ...         ...         ...  5 

„  Allowance  for  Shrinkage  during  ...         ...         ...         28 

„  Crucibles  for        ...         ...         ...         ...         ...         ...         26 

„  Furnace  for          ......         ...          ...         ...          ...         25 

„  Necessity  for  High  Temperature  ..  30 

.,  Petroleum  or  Gas  as  Fuel  for    ...          ...          ...          ...         29 

Pyrometer  for  Use  in 27,29 

of  Various  Forms  of  Filaments ...  ...       21,  23,  25 


214  INDEX   TO    CONTENTS. 

PAGE 

Celluloid,  Filaments  Prepared  from  (Weston)  ...         ...  17 

Cotton  Thread,  Filaments  Prepared  from  (Swan)       ...          ...          ...  7 

Crooke's  Process  for  Making  Filaments  ...          ...          ...          ...         18 

Cuprammonia  Process  for  Making  Filaments  ...          ...         ...          ...         18 

Crucibles  for  Carbonisation        ...          ...          ...          ...          ...          ...          24 

for  Glass  Making        ~     ...    '     104,  105 

Method  of  Packing  Filaments  in 24,25 

„  Unpacking  30,  31 

Curves  Showing  Effect  of  Flashing  on  Resistance  of  Filaments       ...         65 

„   Voltage        „          „  ...         65 

„  „  „  „  ,,    Current       „          „  ...         66 

„  „         Diameter  and  Current  of  Filaments  82,  83 

„  „         Variation  of  Candle-power  with  Variation  of  E.M.F.       198 

„  ,,         Connection  Between  Candle-power  and  Watts      ...        205 

„  „         Relation  of  Logs,  of  Candle-power  to  Logs.  of.  Volts 

or  Watts      207 

Candle-power  and  Volts  Calculated  and  Observed  208,  209 


Deterioration  Tests         ...       196 

Diameters  of  Filaments — see  Filaments,  Sizes  of. 

Draw-plates  for  Cutting  Parchmentised  Thread         ...         ...  ...         15 

Drying  Parchmentised  Thread  ...          ...           ..          ...         ...  13 


Edison,  Bamboo  Filament         ...         ...         ...         ...         ...         ...  19 

,,       Electro-plated  Joint      ...         ...          ...         ...         ...  35 

Edison-Swan  Pin-head  Joint      38 

Efficiency  and  Duration...          ...          ...         ...           ..          ...         ..  195 

Emissivity  of  Different  Varieties  of  Carbon    ..           ...                      ...  4,  63 

Evershed's  Wattmeter 188 

Exhausting  (see  also  Pumps)      ...         ...         ...          ...          ...           ..  131 

„          Removing  Gases  from  Filament  ...         ...          ...          ...  160 

„                 „           Air  from  Surface  of  Glass       ...          ...          ...  162 

„           Precautions  against  Moisture       ...          ...         ...         ...  167 


Filaments,  Errors  in  Thickness  and  Emissivity  of    ...         ...  ...  64 

„                 „          Length  and  Resistance  of          ...         ...  ...  65 

„           Limits  of  Diameter  for       •    ..  ...  87 

„           Made  to  Serve  for  Different  Currents  by  Flashing  ...  60 

„           Measuring     ...          ...          ...         ...         ...  ...  99 

„           Produced  from  Bamboo,  Edison                 ...          ...  ...  19 

„           Produced  from  Cotton,  Swan's  Process    ...          ...  ...  7 

„                    „            „           „        Cuprammonia  Process  (Crookes')  18 

„                   „            „           „        Westou's  Process            ...  ...  17 

„                    „            „           „        Wynne  and  Powell's  Process  ...  16 

Furfurol    .  18 


INDEX    TO    CONTENTS.  215 

PAGE 

Filaments,  Produced  from  Silk  ...          ...          ...  ..          ...         19 

.,  „  „      Vegetable  Parchment      19 

„  Reduced  to  Required  Resistance  by  Flashing     ...         ...         46 

„  Sizes  ,»f,  I'nflashed  69 

Circular  70 

„  „  Square  ...  ..73 

Flat  74 

„  ,  „  Hollow  ...         76 

„       Flashed        ...  81 

„  Spotty,  made  Even  by  Flashing ...         ...         ...         ...         45 

Flashing  45 

„         Object  of  the  Process...          ...          ...          ...         ...          ...47 — 60 

,,         Clip  for  Holding  Filament  during    ...         ...         ...         ...         56 

„         E.M.F.  required  for     ...          ...         ...         ...         ...         ...         61 

„         in  Illuminating  Gas  at  Atmospheric  Pressure        ...         ...         51 

„  ,.         ,.       Reduced  „  54 

in  Gasoline  or  Pentane  Vapour         ...         ...         ...         ...         54 

„  „  „  Vacuum  Connections  for ...         57 

.,         Regulating  Resistance  for  Use  in      ...         ...         ...         ...         53 

„         Thickness  of  Deposit 60 

Resistance  Methods  of  and  Difficulties         ...         ...         ...         49 

Time  Method      .         .  86 

Voltage  Method  61 

Furnace  for  Carbonising...         ...         ...         ...         ...         ...         ...         25 

„  Oil  and  Gas 29 

„          „   Glass  Making          104 

Geiss-ler  Pump 134 

Gimingham's  Mechanical  Mercurial  Pump      ...         ...         ...         ...  176 

Glass-Blowing      109 

„  „         Annealing  ...         ...         ...         ..  123 

,.  „         Blow  Pipes  for 110,  111,   113,   169 

„  „         Bulb  Making      116 

„  „         Precautions  with  Lead  Glass 112 

Sealing-in  121 

„     Making        103 

„     Stoppers,  &c.,  Ground      ..  ...         ...  ..         ...         ...  128 

„     Tubing,  Bulbs  Blown  from          116 

.,  ,.        The  Manufacture  of         108 

.,  „       Method  of  Cutting         127 

Induction  Coil  Test  for  Vacuum  ...          ...          ...          ...  173 


Jamieson's  Law  of  Candle-power 

Joint  between  Leading-in  Wires  and  Filament,  various  types      34,  35,  36 

.,     Butt,  Apparatus  for  Making        ...  40 

,,      of  Edison -Swan  Company...  38 


216  INDEX    TO    CONTENTS. 

PAGE 

Joint,  Carbon,  Deposited  from  Gas       ...          ...  ...  .,           ...  40 

„           „                 „              „      Liquid...  ...  ...          ...  41 

„     Deposited,  Current  required  to  Makp    ...  ...  ...  42 

,,             „          Resistance  for  Use  in  Making  ...  ...          ...  43 

„     Paste           ;..         , 44 

„     Socket,  Apparatus  for  Depositing  Carbon  on  ...  ...          ...  39 


Kennedy's  Pump            ...          ...          ...          .  .          ...         ...          ..  158 

Lane- Fox  Lamp,  Early  Form  of  t  36 

„  „  Pump... ..'  136 

Leading-in  Wires,  Compound    ...         ...         ...         ...         ...         ...  34 

„  „  Platinum  ...  ...  ...  ...  ...  ...  33 

„  „  Size  of  38 

„  „  Spiral  Socket  ..  37 

„  „  Substitutes  for  Platinum 33 

„  Tube  „  37 

Length  of  Filaments — see  Filaments,  Sizes  of. 

Life  Tests             ...          196 

Light  and  Power,  Relation  between 203 

Maxim -Weston  Lamp   ...         ...         ...         ...         ...         ...         ...  36 

McLeod  Vacuum  Gauge 170 

Measuring     ilaments      ...         ...         ...         ...         ...         ...          ...  99 

Mercury,  Cleaning  and  Distilling         ...         ...         ...           .           ...  174 

„         Methods  of  Lifting,  in  Pumps          ...         ...         ...         ...  147 

„         Precautions  in  Using ...         ...         ...         ...          ...          ...  147 

Micrometer  Gauges  for  Measuring  Filaments...         ...          ..,99,  100,  101 

Mineral  Substances  in  Conjunction  with  Carbon  in  Filaments        . .  2 

Moisture,  Absorption  of,  by  Filaments            ...        '...          ...          ...  32 

„         in  Mercurial  Pumps  ...         ...         ...         ..           ..           ...  167 

Mounting  the  Filaments  on  the  Wires            ...         ...         ...         ...  33 

Pentane  Lamp    ...         ..          ...         ...         ...         ...         ...         ...  180 

Pentane  Vapour  Used  for  Flashing      ...         ...         ...         ...         ...  54 

Photometer          •; 181,  183 

Plaster  of  Paris  Used  in  Caps 194 

Platinum  as  a  Filament ...         ...         ...         ...         ...         ...         ...  1 

„         Leading-in  Wires       ...         ...         ...         ...  33 

„         Substitutes  for           ..  33 

Putnps,  Mechanical          ...         ...                     ...         ...         ...         ...  176 

„                „          Gimingham's          ...         ...         ...         ...         ...  176 

„               „          Rotary         177 

„       Mercurial            133 

„              „         Geissler          ...          ...         ...         ...          ...         ...  134 

.              .,         Joining  Lamps  to      .,          ...         ...         ...         ...  168 


INDEX    TO    CONTENTS.  217 

PAGE 

Pumps,  Mercurial,  Kennedy      158 

,,              „          Lane-Fox     136 

»              »           Sprengel,  Simple  Form  of..           144 

„                 „          with  Several  Fall  Tubes           145 

»              „                 „          with  Air-passage  Lift 148 

„              „                 „          Shortened  Form  of       150 

»>              «j                 ,,                „             „       „  due  to  Steam        ...  167 

»»              „           Swinburne  ..           ...         ...         ...         ...         ...  141 

»              »                  >,          Factory  Pattern          ...         ...         ...  155 

Toepler         138 

»>              „          Weston         ...         ...         ...         ...         ...         ...  153 

Pump-Room,  Views  of...  ...  154,  156 

Pyrometer,  for  Use  during  Carbonisation        ...          ...          ...          ...27,  29 

Pyroxyline,  Filaments  prepared  from  (Weston)         ...         ...         ...  17 

Resistance,  Effect  of  Fall  of,  on  the  Candle-power 197,201 

„           Effect  of  Flashing  on         65 

„  Ratio  of  Hot  to  Cold         62,  90,  96 

„           Specific,  of  Carbon 62 

„           Variable,  for  Depositing  on  Joints          ...         ...         ...  43 

„                 „          for  Flashing       53 

Sealing-in  the  Filaments           ...         ...         ...          ...          ...         ...  121 

Shrinkage  of  Filaments  during  Carbonisation             ...          ...         ...  22 

Silk,  Filaments  prepared  from  ...         ...         ...           ..         ...          ...  19 

Spotted  Lamps    ...         ...         ...         ...         ...         ...         ...         ...  125 

Sprengel  Pump,  Simple  Form  of          ...         ...         ...         ...         ...  144 

with  Several  Fall  Tubes        145 

„             „        with  Air  Pressure  Lift           148 

„             „        Shortened  Form         ...         ...          ...         ...         ..  150 

Steam's  Pump 166,167 

Surface  Area  of  Filaments         ...         ...         ...         ..           ...         ...  64 

Swan,  Parchmentised  Thread  Process ...          ...         ...         ...         ...  7 

,,                   „                   „             „       Strength  of  Acid  used          ...  11 

„                   „          .        „             „       Drying  the  Thread    ...         ...  13 

„                  „                  „            „       Cutting  to  Size          ...         ...  15 

„      Lamp,  Early  Form  of      ..           ..           ...         ...         35 

Swinburne,  Arrangements  for  Constantly  Maintaining  a  Vacuum  in 

a  Pump 163 

„          Method  of  Testing  Lamps 182 

„          Pump            141 

,,               „      Factory  Pattern      I".". 

Temperature,  Effect  of,  on  Filaments  during  Carbonisation           ...  30 

Testing  Lamps     ...         ...         ...         ...         ...         ...  179 

Details  of           183 

Electrical  Connections  for       ...          ...         ...         ...  187 


218  INDEX    TO    CONTENTS 

PAGE 

Testing  Lamps,  Light  Standard  for      ...         ...         ...         ...         ...  180 

„             „       Photometer  for            ».         181 

Wattmeter  for...         ..  ^        186 

„         Vacuum...         ...         ...         ...     ......         ...         ...         ...  169 

„       McLeod  Gauge         170 

by  Induction  Coil 173 

Thompson,  Prof.  S.  P.,  on  Mercurial  Air-Pumps         ..         ...         ...  133 

„              „         „      on  Volatilisation  of  Carbon   ..           ...          ...  2 

Toepler's  Pump 138 

Torricellian  Vacuum       ...         ...         ...         ...         ...         ...         ...  133 

Trotter's  Micrometer  Gauge      100 

Vacuum  in  Lamps,  Necessity  for         ...         ...         ...         ...         ...  131 

„        Pumps— see  Pumps. 

„        Testing — see  Testing. 

Volatilisation  of  Carbon...         ...         ...         ...         ...          ...         ...  2,  3 


Wattmeter,  Evershed's 188 

„             Necessity  for         180 

Welsbach  Gas  Light       195 

Weston's  Filament          17 

„         Lamp 36 

Pump 153 

Wynne  and  Powell's  Filament ...         16,  17 


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